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perf: Fix segbase for ld.lld linked objects

PATCH Applied Fangrui Song 4 participants 6 messages May 27, 2022 – Jun 3, 2022 perf events

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Fangrui Song

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#0 lore raw

PATCH v2 1/1 perf: Fix segbase for ld.lld linked objects

segbase is the address of .eh_frame_hdr and table_data is segbase plus the header size. find_proc_info computes segbase as map->start + segbase - map->pgoff which is wrong when

* .eh_frame_hdr and .text are in different PT_LOAD program headers
* and their p_vaddr difference does not equal their p_offset difference

Since 10.0, ld.lld's default --rosegment -z noseparate-code layout has such R and RX PT_LOAD program headers.

ld.lld (default) => perf report fails to unwind perf record --call-graph dwarf recorded data
ld.lld --no-rosegment => ok (trivial, no R PT_LOAD) ld.lld -z separate-code => ok but by luck: there are two PT_LOAD but their p_vaddr difference equals p_offset difference

ld.bfd -z noseparate-code => ok (trivial, no R PT_LOAD) ld.bfd -z separate-code (default for Linux/x86) => ok but by luck:
there are two PT_LOAD but their p_vaddr difference equals p_offset difference

To fix the issue, compute segbase as dso's base address plus PT_GNU_EH_FRAME's p_vaddr. The base address is computed by iterating over all dso-associated maps and then subtract the first PT_LOAD p_vaddr (the minimum guaranteed by generic ABI) from the minimum address.

In libunwind, find_proc_info transitively called by unw_step is cached, so the iteration overhead is acceptable.

Reported-by Sebastian Ullrich <[email protected]>
Link https://github.com/ClangBuiltLinux/linux/issues/1646
Signed-off-by Fangrui Song <[email protected]>
Cc Ian Rogers <[email protected]>

--
Changes from v1:
* Fix elf_base_address to use the first PT_LOAD
* Use dso::elf_base_addr which is a constant even if the dso is loaded into multiple processes

---
 tools/perf/util/dso.h                    |   2 +
 tools/perf/util/unwind-libunwind-local.c | 107 ++++++++++++++++-------
 2 files changed, 78 insertions(+), 31 deletions(-)
tools/perf/util/dso.h +2 -0
index 3a9fd4d389b5..97047a11282b 100644
@@ -196,7 +196,9 @@ struct dso {
 		u32		 status_seen;
 		u64		 file_size;
 		struct list_head open_entry;
+		u64		 elf_base_addr;
 		u64		 debug_frame_offset;
+		u64		 eh_frame_hdr_addr;
 		u64		 eh_frame_hdr_offset;
 	} data;
 	/* bpf prog information */
tools/perf/util/unwind-libunwind-local.c +76 -31

163 lines not on this page · raw message

Signature · 1 line
2.36.1.124.g0e6072fb45-goog
reviewer Reviewed-by on v2 1/1
#1 lore raw
Quoted 27 lines · Fangrui Song

segbase is the address of .eh_frame_hdr and table_data is segbase plus the header size. find_proc_info computes segbase as map->start + segbase - map->pgoff which is wrong when

* .eh_frame_hdr and .text are in different PT_LOAD program headers
* and their p_vaddr difference does not equal their p_offset difference

Since 10.0, ld.lld's default --rosegment -z noseparate-code layout has such R and RX PT_LOAD program headers.

ld.lld (default) => perf report fails to unwind perf record --call-graph dwarf recorded data
ld.lld --no-rosegment => ok (trivial, no R PT_LOAD) ld.lld -z separate-code => ok but by luck: there are two PT_LOAD but their p_vaddr difference equals p_offset difference

ld.bfd -z noseparate-code => ok (trivial, no R PT_LOAD) ld.bfd -z separate-code (default for Linux/x86) => ok but by luck:
there are two PT_LOAD but their p_vaddr difference equals p_offset difference

To fix the issue, compute segbase as dso's base address plus PT_GNU_EH_FRAME's p_vaddr. The base address is computed by iterating over all dso-associated maps and then subtract the first PT_LOAD p_vaddr (the minimum guaranteed by generic ABI) from the minimum address.

Quoted 7 lines

In libunwind, find_proc_info transitively called by unw_step is cached, so the iteration overhead is acceptable.

Reported-by Sebastian Ullrich <[email protected]>
Link https://github.com/ClangBuiltLinux/linux/issues/1646
Signed-off-by Fangrui Song <[email protected]>
Cc Ian Rogers <[email protected]>
Reviewed-by Ian Rogers <[email protected]>

Thanks!
Ian

Quoted 197 lines

--
Changes from v1:
* Fix elf_base_address to use the first PT_LOAD
* Use dso::elf_base_addr which is a constant even if the dso is loaded into multiple processes

---
 tools/perf/util/dso.h                    |   2 +
 tools/perf/util/unwind-libunwind-local.c | 107 ++++++++++++++++-------
 2 files changed, 78 insertions(+), 31 deletions(-)
tools/perf/util/dso.h +2 -0
index 3a9fd4d389b5..97047a11282b 100644
@@ -196,7 +196,9 @@ struct dso {
                u32              status_seen;
                u64              file_size;
                struct list_head open_entry;
+               u64              elf_base_addr;
                u64              debug_frame_offset;
+               u64              eh_frame_hdr_addr;
                u64              eh_frame_hdr_offset;
        } data;
        /* bpf prog information */
tools/perf/util/unwind-libunwind-local.c +76 -32
index 41e29fc7648a..37622699c91a 100644
@@ -169,30 +169,64 @@ static int __dw_read_encoded_value(u8 **p, u8 *end, u64 *val,
        __v;                                                    \
        })
 
-static u64 elf_section_offset(int fd, const char *name)
+static int elf_section_address_and_offset(int fd, const char *name, u64 *address, u64 *offset)
 {
        Elf *elf;
        GElf_Ehdr ehdr;
        GElf_Shdr shdr;
-       u64 offset = 0;
+       int ret;
 
        elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
        if (elf == NULL)
-               return 0;
+               return -1;
 
-       do {
-               if (gelf_getehdr(elf, &ehdr) == NULL)
-                       break;
+       if (gelf_getehdr(elf, &ehdr) == NULL)
+               goto out_err;
 
-               if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
-                       break;
-
-               offset = shdr.sh_offset;
-       } while (0);
+       if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
+               goto out_err;
 
+       *address = shdr.sh_addr;
+       *offset = shdr.sh_offset;
+       ret = 0;
+out_err:
        elf_end(elf);
+       return ret;
+}
+
+#ifndef NO_LIBUNWIND_DEBUG_FRAME
+static u64 elf_section_offset(int fd, const char *name)
+{
+       u64 address, offset;
+
+       if (elf_section_address_and_offset(fd, name, &address, &offset))
+               return 0;
+
        return offset;
 }
+#endif
+
+static u64 elf_base_address(int fd)
+{
+       Elf *elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
+       GElf_Phdr phdr;
+       u64 retval = 0;
+       size_t i, phdrnum = 0;
+
+       if (elf == NULL)
+               return 0;
+       (void)elf_getphdrnum(elf, &phdrnum);
+       /* PT_LOAD segments are sorted by p_vaddr, so the first has the minimum p_vaddr. */
+       for (i = 0; i < phdrnum; i++) {
+               if (gelf_getphdr(elf, i, &phdr) && phdr.p_type == PT_LOAD) {
+                       retval = phdr.p_vaddr & -getpagesize();
+                       break;
+               }
+       }
+
+       elf_end(elf);
+       return retval;
+}
 
 #ifndef NO_LIBUNWIND_DEBUG_FRAME
 static int elf_is_exec(int fd, const char *name)

@@ -248,8 +282,7 @@ struct eh_frame_hdr {
 } __packed;
 
 static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
-                              u64 offset, u64 *table_data, u64 *segbase,
-                              u64 *fde_count)
+                              u64 offset, u64 *table_data_offset, u64 *fde_count)
 {
        struct eh_frame_hdr hdr;
        u8 *enc = (u8 *) &hdr.enc;

@@ -265,35 +298,47 @@ static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
        dw_read_encoded_value(enc, end, hdr.eh_frame_ptr_enc);
 
        *fde_count  = dw_read_encoded_value(enc, end, hdr.fde_count_enc);
-       *segbase    = offset;
-       *table_data = (enc - (u8 *) &hdr) + offset;
+       *table_data_offset = enc - (u8 *) &hdr;
        return 0;
 }
 
-static int read_unwind_spec_eh_frame(struct dso *dso, struct machine *machine,
+static int read_unwind_spec_eh_frame(struct dso *dso, struct unwind_info *ui,
                                     u64 *table_data, u64 *segbase,
                                     u64 *fde_count)
 {
-       int ret = -EINVAL, fd;
-       u64 offset = dso->data.eh_frame_hdr_offset;
+       struct map *map;
+       u64 base_addr = UINT64_MAX;
+       int ret, fd;
 
-       if (offset == 0) {
-               fd = dso__data_get_fd(dso, machine);
+       if (dso->data.eh_frame_hdr_offset == 0) {
+               fd = dso__data_get_fd(dso, ui->machine);
                if (fd < 0)
                        return -EINVAL;
 
                /* Check the .eh_frame section for unwinding info */
-               offset = elf_section_offset(fd, ".eh_frame_hdr");
-               dso->data.eh_frame_hdr_offset = offset;
+               ret = elf_section_address_and_offset(fd, ".eh_frame_hdr",
+                                                    &dso->data.eh_frame_hdr_addr,
+                                                    &dso->data.eh_frame_hdr_offset);
+               dso->data.elf_base_addr = elf_base_address(fd);
                dso__data_put_fd(dso);
+               if (ret || dso->data.eh_frame_hdr_offset == 0)
+                       return -EINVAL;
        }
 
-       if (offset)
-               ret = unwind_spec_ehframe(dso, machine, offset,
-                                         table_data, segbase,
-                                         fde_count);
-
-       return ret;
+       maps__for_each_entry(ui->thread->maps, map) {
+               if (map->dso == dso && map->start < base_addr)
+                       base_addr = map->start;
+       }
+       base_addr -= dso->data.elf_base_addr;
+       /* Address of .eh_frame_hdr */
+       *segbase = base_addr + dso->data.eh_frame_hdr_addr;
+       ret = unwind_spec_ehframe(dso, ui->machine, dso->data.eh_frame_hdr_offset,
+                                  table_data, fde_count);
+       if (ret)
+               return ret;
+       /* binary_search_table offset plus .eh_frame_hdr address */
+       *table_data += *segbase;
+       return 0;
 }
 
 #ifndef NO_LIBUNWIND_DEBUG_FRAME

@@ -388,14 +433,14 @@ find_proc_info(unw_addr_space_t as, unw_word_t ip, unw_proc_info_t *pi,
        pr_debug("unwind: find_proc_info dso %s\n", map->dso->name);
 
        /* Check the .eh_frame section for unwinding info */
-       if (!read_unwind_spec_eh_frame(map->dso, ui->machine,
+       if (!read_unwind_spec_eh_frame(map->dso, ui,
                                       &table_data, &segbase, &fde_count)) {
                memset(&di, 0, sizeof(di));
                di.format   = UNW_INFO_FORMAT_REMOTE_TABLE;
                di.start_ip = map->start;
                di.end_ip   = map->end;
-               di.u.rti.segbase    = map->start + segbase - map->pgoff;
-               di.u.rti.table_data = map->start + table_data - map->pgoff;
+               di.u.rti.segbase    = segbase;
+               di.u.rti.table_data = table_data;
                di.u.rti.table_len  = fde_count * sizeof(struct table_entry)
                                      / sizeof(unw_word_t);
                ret = dwarf_search_unwind_table(as, ip, &di, pi,
--

2.36.1.124.g0e6072fb45-goog

maintainer Applied
#2 lore raw

Em Fri, May 27, 2022 at 11:51:54AM -0700, Ian Rogers escreveu:

Quoted 31 lines · Fangrui Song
Quoted 30 lines

segbase is the address of .eh_frame_hdr and table_data is segbase plus the header size. find_proc_info computes segbase as map->start + segbase - map->pgoff which is wrong when

* .eh_frame_hdr and .text are in different PT_LOAD program headers
* and their p_vaddr difference does not equal their p_offset difference

Since 10.0, ld.lld's default --rosegment -z noseparate-code layout has such R and RX PT_LOAD program headers.

ld.lld (default) => perf report fails to unwind perf record --call-graph dwarf recorded data
ld.lld --no-rosegment => ok (trivial, no R PT_LOAD) ld.lld -z separate-code => ok but by luck: there are two PT_LOAD but their p_vaddr difference equals p_offset difference

ld.bfd -z noseparate-code => ok (trivial, no R PT_LOAD) ld.bfd -z separate-code (default for Linux/x86) => ok but by luck:
there are two PT_LOAD but their p_vaddr difference equals p_offset difference

To fix the issue, compute segbase as dso's base address plus PT_GNU_EH_FRAME's p_vaddr. The base address is computed by iterating over all dso-associated maps and then subtract the first PT_LOAD p_vaddr (the minimum guaranteed by generic ABI) from the minimum address.

In libunwind, find_proc_info transitively called by unw_step is cached, so the iteration overhead is acceptable.

Quoted 6 lines
Quoted 4 lines
Reported-by Sebastian Ullrich <[email protected]>
Link https://github.com/ClangBuiltLinux/linux/issues/1646
Signed-off-by Fangrui Song <[email protected]>
Cc Ian Rogers <[email protected]>
Reviewed-by Ian Rogers <[email protected]>

Thanks, applied.

- Arnaldo

#3 lore raw
In reply to Fangrui Song
Quoted 26 lines · Fangrui Song

segbase is the address of .eh_frame_hdr and table_data is segbase plus the header size. find_proc_info computes segbase as map->start + segbase - map->pgoff which is wrong when

* .eh_frame_hdr and .text are in different PT_LOAD program headers
* and their p_vaddr difference does not equal their p_offset difference

Since 10.0, ld.lld's default --rosegment -z noseparate-code layout has such R and RX PT_LOAD program headers.

ld.lld (default) => perf report fails to unwind perf record --call-graph dwarf recorded data
ld.lld --no-rosegment => ok (trivial, no R PT_LOAD) ld.lld -z separate-code => ok but by luck: there are two PT_LOAD but their p_vaddr difference equals p_offset difference

ld.bfd -z noseparate-code => ok (trivial, no R PT_LOAD) ld.bfd -z separate-code (default for Linux/x86) => ok but by luck:
there are two PT_LOAD but their p_vaddr difference equals p_offset difference

To fix the issue, compute segbase as dso's base address plus PT_GNU_EH_FRAME's p_vaddr. The base address is computed by iterating over all dso-associated maps and then subtract the first PT_LOAD p_vaddr (the minimum guaranteed by generic ABI) from the minimum address.

Quoted 7 lines

In libunwind, find_proc_info transitively called by unw_step is cached, so the iteration overhead is acceptable.

Reported-by Sebastian Ullrich <[email protected]>
Link https://github.com/ClangBuiltLinux/linux/issues/1646
Signed-off-by Fangrui Song <[email protected]>
Cc Ian Rogers <[email protected]>

nice, is this a problem for libdw dwarf unwind as well?

you should be able to build it with:

  $ make NO_LIBUNWIND=1

thanks,
jirka

Quoted 198 lines

--
Changes from v1:
* Fix elf_base_address to use the first PT_LOAD
* Use dso::elf_base_addr which is a constant even if the dso is loaded into multiple processes

---
 tools/perf/util/dso.h                    |   2 +
 tools/perf/util/unwind-libunwind-local.c | 107 ++++++++++++++++-------
 2 files changed, 78 insertions(+), 31 deletions(-)
tools/perf/util/dso.h +2 -0
index 3a9fd4d389b5..97047a11282b 100644
@@ -196,7 +196,9 @@ struct dso {
 		u32		 status_seen;
 		u64		 file_size;
 		struct list_head open_entry;
+		u64		 elf_base_addr;
 		u64		 debug_frame_offset;
+		u64		 eh_frame_hdr_addr;
 		u64		 eh_frame_hdr_offset;
 	} data;
 	/* bpf prog information */
tools/perf/util/unwind-libunwind-local.c +76 -31
index 41e29fc7648a..37622699c91a 100644
@@ -169,30 +169,64 @@ static int __dw_read_encoded_value(u8 **p, u8 *end, u64 *val,
 	__v;                                                    \
 	})
 
-static u64 elf_section_offset(int fd, const char *name)
+static int elf_section_address_and_offset(int fd, const char *name, u64 *address, u64 *offset)
 {
 	Elf *elf;
 	GElf_Ehdr ehdr;
 	GElf_Shdr shdr;
-	u64 offset = 0;
+	int ret;
 
 	elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
 	if (elf == NULL)
-		return 0;
+		return -1;
 
-	do {
-		if (gelf_getehdr(elf, &ehdr) == NULL)
-			break;
+	if (gelf_getehdr(elf, &ehdr) == NULL)
+		goto out_err;
 
-		if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
-			break;
-
-		offset = shdr.sh_offset;
-	} while (0);
+	if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
+		goto out_err;
 
+	*address = shdr.sh_addr;
+	*offset = shdr.sh_offset;
+	ret = 0;
+out_err:
 	elf_end(elf);
+	return ret;
+}
+
+#ifndef NO_LIBUNWIND_DEBUG_FRAME
+static u64 elf_section_offset(int fd, const char *name)
+{
+	u64 address, offset;
+
+	if (elf_section_address_and_offset(fd, name, &address, &offset))
+		return 0;
+
 	return offset;
 }
+#endif
+
+static u64 elf_base_address(int fd)
+{
+	Elf *elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
+	GElf_Phdr phdr;
+	u64 retval = 0;
+	size_t i, phdrnum = 0;
+
+	if (elf == NULL)
+		return 0;
+	(void)elf_getphdrnum(elf, &phdrnum);
+	/* PT_LOAD segments are sorted by p_vaddr, so the first has the minimum p_vaddr. */
+	for (i = 0; i < phdrnum; i++) {
+		if (gelf_getphdr(elf, i, &phdr) && phdr.p_type == PT_LOAD) {
+			retval = phdr.p_vaddr & -getpagesize();
+			break;
+		}
+	}
+
+	elf_end(elf);
+	return retval;
+}
 
 #ifndef NO_LIBUNWIND_DEBUG_FRAME
 static int elf_is_exec(int fd, const char *name)

@@ -248,8 +282,7 @@ struct eh_frame_hdr {
 } __packed;
 
 static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
-			       u64 offset, u64 *table_data, u64 *segbase,
-			       u64 *fde_count)
+			       u64 offset, u64 *table_data_offset, u64 *fde_count)
 {
 	struct eh_frame_hdr hdr;
 	u8 *enc = (u8 *) &hdr.enc;

@@ -265,35 +298,47 @@ static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
 	dw_read_encoded_value(enc, end, hdr.eh_frame_ptr_enc);
 
 	*fde_count  = dw_read_encoded_value(enc, end, hdr.fde_count_enc);
-	*segbase    = offset;
-	*table_data = (enc - (u8 *) &hdr) + offset;
+	*table_data_offset = enc - (u8 *) &hdr;
 	return 0;
 }
 
-static int read_unwind_spec_eh_frame(struct dso *dso, struct machine *machine,
+static int read_unwind_spec_eh_frame(struct dso *dso, struct unwind_info *ui,
 				     u64 *table_data, u64 *segbase,
 				     u64 *fde_count)
 {
-	int ret = -EINVAL, fd;
-	u64 offset = dso->data.eh_frame_hdr_offset;
+	struct map *map;
+	u64 base_addr = UINT64_MAX;
+	int ret, fd;
 
-	if (offset == 0) {
-		fd = dso__data_get_fd(dso, machine);
+	if (dso->data.eh_frame_hdr_offset == 0) {
+		fd = dso__data_get_fd(dso, ui->machine);
 		if (fd < 0)
 			return -EINVAL;
 
 		/* Check the .eh_frame section for unwinding info */
-		offset = elf_section_offset(fd, ".eh_frame_hdr");
-		dso->data.eh_frame_hdr_offset = offset;
+		ret = elf_section_address_and_offset(fd, ".eh_frame_hdr",
+						     &dso->data.eh_frame_hdr_addr,
+						     &dso->data.eh_frame_hdr_offset);
+		dso->data.elf_base_addr = elf_base_address(fd);
 		dso__data_put_fd(dso);
+		if (ret || dso->data.eh_frame_hdr_offset == 0)
+			return -EINVAL;
 	}
 
-	if (offset)
-		ret = unwind_spec_ehframe(dso, machine, offset,
-					  table_data, segbase,
-					  fde_count);
-
-	return ret;
+	maps__for_each_entry(ui->thread->maps, map) {
+		if (map->dso == dso && map->start < base_addr)
+			base_addr = map->start;
+	}
+	base_addr -= dso->data.elf_base_addr;
+	/* Address of .eh_frame_hdr */
+	*segbase = base_addr + dso->data.eh_frame_hdr_addr;
+	ret = unwind_spec_ehframe(dso, ui->machine, dso->data.eh_frame_hdr_offset,
+				   table_data, fde_count);
+	if (ret)
+		return ret;
+	/* binary_search_table offset plus .eh_frame_hdr address */
+	*table_data += *segbase;
+	return 0;
 }
 
 #ifndef NO_LIBUNWIND_DEBUG_FRAME

@@ -388,14 +433,14 @@ find_proc_info(unw_addr_space_t as, unw_word_t ip, unw_proc_info_t *pi,
 	pr_debug("unwind: find_proc_info dso %s\n", map->dso->name);
 
 	/* Check the .eh_frame section for unwinding info */
-	if (!read_unwind_spec_eh_frame(map->dso, ui->machine,
+	if (!read_unwind_spec_eh_frame(map->dso, ui,
 				       &table_data, &segbase, &fde_count)) {
 		memset(&di, 0, sizeof(di));
 		di.format   = UNW_INFO_FORMAT_REMOTE_TABLE;
 		di.start_ip = map->start;
 		di.end_ip   = map->end;
-		di.u.rti.segbase    = map->start + segbase - map->pgoff;
-		di.u.rti.table_data = map->start + table_data - map->pgoff;
+		di.u.rti.segbase    = segbase;
+		di.u.rti.table_data = table_data;
 		di.u.rti.table_len  = fde_count * sizeof(struct table_entry)
 				      / sizeof(unw_word_t);
 		ret = dwarf_search_unwind_table(as, ip, &di, pi,
Signature · 1 line
2.36.1.124.g0e6072fb45-goog
reviewer
#4 lore raw
Quoted 31 lines · Jiri Olsa
Quoted 29 lines · Fangrui Song

segbase is the address of .eh_frame_hdr and table_data is segbase plus the header size. find_proc_info computes segbase as map->start + segbase - map->pgoff which is wrong when

* .eh_frame_hdr and .text are in different PT_LOAD program headers
* and their p_vaddr difference does not equal their p_offset difference

Since 10.0, ld.lld's default --rosegment -z noseparate-code layout has such R and RX PT_LOAD program headers.

ld.lld (default) => perf report fails to unwind perf record --call-graph dwarf recorded data
ld.lld --no-rosegment => ok (trivial, no R PT_LOAD) ld.lld -z separate-code => ok but by luck: there are two PT_LOAD but their p_vaddr difference equals p_offset difference

ld.bfd -z noseparate-code => ok (trivial, no R PT_LOAD) ld.bfd -z separate-code (default for Linux/x86) => ok but by luck:
there are two PT_LOAD but their p_vaddr difference equals p_offset difference

To fix the issue, compute segbase as dso's base address plus PT_GNU_EH_FRAME's p_vaddr. The base address is computed by iterating over all dso-associated maps and then subtract the first PT_LOAD p_vaddr (the minimum guaranteed by generic ABI) from the minimum address.

In libunwind, find_proc_info transitively called by unw_step is cached, so the iteration overhead is acceptable.

Quoted 6 lines
Quoted 4 lines
Reported-by Sebastian Ullrich <[email protected]>
Link https://github.com/ClangBuiltLinux/linux/issues/1646
Signed-off-by Fangrui Song <[email protected]>
Cc Ian Rogers <[email protected]>

nice, is this a problem for libdw dwarf unwind as well?

libdw also is broken with lld but not bfd. When I switch on verbose output (-vv) with perf report I see:

overlapping maps:
202000-203000 0 a.out
202000-203000 0 a.out

and:

unwind: failed with 'address range overlaps an existing module'

The sense I get from this is libdw is rejecting the file before anything we can do in perf.

Thanks,
Ian

Quoted 205 lines

you should be able to build it with:

  $ make NO_LIBUNWIND=1

thanks,
jirka

Quoted 198 lines

--
Changes from v1:
* Fix elf_base_address to use the first PT_LOAD
* Use dso::elf_base_addr which is a constant even if the dso is loaded into multiple processes

---
 tools/perf/util/dso.h                    |   2 +
 tools/perf/util/unwind-libunwind-local.c | 107 ++++++++++++++++-------
 2 files changed, 78 insertions(+), 31 deletions(-)
tools/perf/util/dso.h +2 -0
index 3a9fd4d389b5..97047a11282b 100644
@@ -196,7 +196,9 @@ struct dso {
              u32              status_seen;
              u64              file_size;
              struct list_head open_entry;
+             u64              elf_base_addr;
              u64              debug_frame_offset;
+             u64              eh_frame_hdr_addr;
              u64              eh_frame_hdr_offset;
      } data;
      /* bpf prog information */
tools/perf/util/unwind-libunwind-local.c +76 -32
index 41e29fc7648a..37622699c91a 100644
@@ -169,30 +169,64 @@ static int __dw_read_encoded_value(u8 **p, u8 *end, u64 *val,
      __v;                                                    \
      })
 
-static u64 elf_section_offset(int fd, const char *name)
+static int elf_section_address_and_offset(int fd, const char *name, u64 *address, u64 *offset)
 {
      Elf *elf;
      GElf_Ehdr ehdr;
      GElf_Shdr shdr;
-     u64 offset = 0;
+     int ret;
 
      elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
      if (elf == NULL)
-             return 0;
+             return -1;
 
-     do {
-             if (gelf_getehdr(elf, &ehdr) == NULL)
-                     break;
+     if (gelf_getehdr(elf, &ehdr) == NULL)
+             goto out_err;
 
-             if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
-                     break;
-
-             offset = shdr.sh_offset;
-     } while (0);
+     if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
+             goto out_err;
 
+     *address = shdr.sh_addr;
+     *offset = shdr.sh_offset;
+     ret = 0;
+out_err:
      elf_end(elf);
+     return ret;
+}
+
+#ifndef NO_LIBUNWIND_DEBUG_FRAME
+static u64 elf_section_offset(int fd, const char *name)
+{
+     u64 address, offset;
+
+     if (elf_section_address_and_offset(fd, name, &address, &offset))
+             return 0;
+
      return offset;
 }
+#endif
+
+static u64 elf_base_address(int fd)
+{
+     Elf *elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
+     GElf_Phdr phdr;
+     u64 retval = 0;
+     size_t i, phdrnum = 0;
+
+     if (elf == NULL)
+             return 0;
+     (void)elf_getphdrnum(elf, &phdrnum);
+     /* PT_LOAD segments are sorted by p_vaddr, so the first has the minimum p_vaddr. */
+     for (i = 0; i < phdrnum; i++) {
+             if (gelf_getphdr(elf, i, &phdr) && phdr.p_type == PT_LOAD) {
+                     retval = phdr.p_vaddr & -getpagesize();
+                     break;
+             }
+     }
+
+     elf_end(elf);
+     return retval;
+}
 
 #ifndef NO_LIBUNWIND_DEBUG_FRAME
 static int elf_is_exec(int fd, const char *name)

@@ -248,8 +282,7 @@ struct eh_frame_hdr {
 } __packed;
 
 static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
-                            u64 offset, u64 *table_data, u64 *segbase,
-                            u64 *fde_count)
+                            u64 offset, u64 *table_data_offset, u64 *fde_count)
 {
      struct eh_frame_hdr hdr;
      u8 *enc = (u8 *) &hdr.enc;

@@ -265,35 +298,47 @@ static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
      dw_read_encoded_value(enc, end, hdr.eh_frame_ptr_enc);
 
      *fde_count  = dw_read_encoded_value(enc, end, hdr.fde_count_enc);
-     *segbase    = offset;
-     *table_data = (enc - (u8 *) &hdr) + offset;
+     *table_data_offset = enc - (u8 *) &hdr;
      return 0;
 }
 
-static int read_unwind_spec_eh_frame(struct dso *dso, struct machine *machine,
+static int read_unwind_spec_eh_frame(struct dso *dso, struct unwind_info *ui,
                                   u64 *table_data, u64 *segbase,
                                   u64 *fde_count)
 {
-     int ret = -EINVAL, fd;
-     u64 offset = dso->data.eh_frame_hdr_offset;
+     struct map *map;
+     u64 base_addr = UINT64_MAX;
+     int ret, fd;
 
-     if (offset == 0) {
-             fd = dso__data_get_fd(dso, machine);
+     if (dso->data.eh_frame_hdr_offset == 0) {
+             fd = dso__data_get_fd(dso, ui->machine);
              if (fd < 0)
                      return -EINVAL;
 
              /* Check the .eh_frame section for unwinding info */
-             offset = elf_section_offset(fd, ".eh_frame_hdr");
-             dso->data.eh_frame_hdr_offset = offset;
+             ret = elf_section_address_and_offset(fd, ".eh_frame_hdr",
+                                                  &dso->data.eh_frame_hdr_addr,
+                                                  &dso->data.eh_frame_hdr_offset);
+             dso->data.elf_base_addr = elf_base_address(fd);
              dso__data_put_fd(dso);
+             if (ret || dso->data.eh_frame_hdr_offset == 0)
+                     return -EINVAL;
      }
 
-     if (offset)
-             ret = unwind_spec_ehframe(dso, machine, offset,
-                                       table_data, segbase,
-                                       fde_count);
-
-     return ret;
+     maps__for_each_entry(ui->thread->maps, map) {
+             if (map->dso == dso && map->start < base_addr)
+                     base_addr = map->start;
+     }
+     base_addr -= dso->data.elf_base_addr;
+     /* Address of .eh_frame_hdr */
+     *segbase = base_addr + dso->data.eh_frame_hdr_addr;
+     ret = unwind_spec_ehframe(dso, ui->machine, dso->data.eh_frame_hdr_offset,
+                                table_data, fde_count);
+     if (ret)
+             return ret;
+     /* binary_search_table offset plus .eh_frame_hdr address */
+     *table_data += *segbase;
+     return 0;
 }
 
 #ifndef NO_LIBUNWIND_DEBUG_FRAME

@@ -388,14 +433,14 @@ find_proc_info(unw_addr_space_t as, unw_word_t ip, unw_proc_info_t *pi,
      pr_debug("unwind: find_proc_info dso %s\n", map->dso->name);
 
      /* Check the .eh_frame section for unwinding info */
-     if (!read_unwind_spec_eh_frame(map->dso, ui->machine,
+     if (!read_unwind_spec_eh_frame(map->dso, ui,
                                     &table_data, &segbase, &fde_count)) {
              memset(&di, 0, sizeof(di));
              di.format   = UNW_INFO_FORMAT_REMOTE_TABLE;
              di.start_ip = map->start;
              di.end_ip   = map->end;
-             di.u.rti.segbase    = map->start + segbase - map->pgoff;
-             di.u.rti.table_data = map->start + table_data - map->pgoff;
+             di.u.rti.segbase    = segbase;
+             di.u.rti.table_data = table_data;
              di.u.rti.table_len  = fde_count * sizeof(struct table_entry)
                                    / sizeof(unw_word_t);
              ret = dwarf_search_unwind_table(as, ip, &di, pi,
--

2.36.1.124.g0e6072fb45-goog

#5 lore raw
Quoted 46 lines · Ian Rogers
Quoted 31 lines · Jiri Olsa
Quoted 29 lines · Fangrui Song

segbase is the address of .eh_frame_hdr and table_data is segbase plus the header size. find_proc_info computes segbase as map->start + segbase - map->pgoff which is wrong when

* .eh_frame_hdr and .text are in different PT_LOAD program headers
* and their p_vaddr difference does not equal their p_offset difference

Since 10.0, ld.lld's default --rosegment -z noseparate-code layout has such R and RX PT_LOAD program headers.

ld.lld (default) => perf report fails to unwind perf record --call-graph dwarf recorded data
ld.lld --no-rosegment => ok (trivial, no R PT_LOAD) ld.lld -z separate-code => ok but by luck: there are two PT_LOAD but their p_vaddr difference equals p_offset difference

ld.bfd -z noseparate-code => ok (trivial, no R PT_LOAD) ld.bfd -z separate-code (default for Linux/x86) => ok but by luck:
there are two PT_LOAD but their p_vaddr difference equals p_offset difference

To fix the issue, compute segbase as dso's base address plus PT_GNU_EH_FRAME's p_vaddr. The base address is computed by iterating over all dso-associated maps and then subtract the first PT_LOAD p_vaddr (the minimum guaranteed by generic ABI) from the minimum address.

In libunwind, find_proc_info transitively called by unw_step is cached, so the iteration overhead is acceptable.

Quoted 6 lines
Quoted 4 lines
Reported-by Sebastian Ullrich <[email protected]>
Link https://github.com/ClangBuiltLinux/linux/issues/1646
Signed-off-by Fangrui Song <[email protected]>
Cc Ian Rogers <[email protected]>

nice, is this a problem for libdw dwarf unwind as well?

libdw also is broken with lld but not bfd. When I switch on verbose output (-vv) with perf report I see:

overlapping maps:
202000-203000 0 a.out
202000-203000 0 a.out

Quoted 6 lines

and:

unwind: failed with 'address range overlaps an existing module'

The sense I get from this is libdw is rejecting the file before anything we can do in perf.

Jan,
any idea if can this be fixed in libdw?

thanks,
jirka

Quoted 209 lines

Thanks,
Ian

Quoted 205 lines

you should be able to build it with:

  $ make NO_LIBUNWIND=1

thanks,
jirka

Quoted 198 lines

--
Changes from v1:
* Fix elf_base_address to use the first PT_LOAD
* Use dso::elf_base_addr which is a constant even if the dso is loaded into multiple processes

---
 tools/perf/util/dso.h                    |   2 +
 tools/perf/util/unwind-libunwind-local.c | 107 ++++++++++++++++-------
 2 files changed, 78 insertions(+), 31 deletions(-)
tools/perf/util/dso.h +2 -0
index 3a9fd4d389b5..97047a11282b 100644
@@ -196,7 +196,9 @@ struct dso {
              u32              status_seen;
              u64              file_size;
              struct list_head open_entry;
+             u64              elf_base_addr;
              u64              debug_frame_offset;
+             u64              eh_frame_hdr_addr;
              u64              eh_frame_hdr_offset;
      } data;
      /* bpf prog information */
tools/perf/util/unwind-libunwind-local.c +76 -32
index 41e29fc7648a..37622699c91a 100644
@@ -169,30 +169,64 @@ static int __dw_read_encoded_value(u8 **p, u8 *end, u64 *val,
      __v;                                                    \
      })
 
-static u64 elf_section_offset(int fd, const char *name)
+static int elf_section_address_and_offset(int fd, const char *name, u64 *address, u64 *offset)
 {
      Elf *elf;
      GElf_Ehdr ehdr;
      GElf_Shdr shdr;
-     u64 offset = 0;
+     int ret;
 
      elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
      if (elf == NULL)
-             return 0;
+             return -1;
 
-     do {
-             if (gelf_getehdr(elf, &ehdr) == NULL)
-                     break;
+     if (gelf_getehdr(elf, &ehdr) == NULL)
+             goto out_err;
 
-             if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
-                     break;
-
-             offset = shdr.sh_offset;
-     } while (0);
+     if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
+             goto out_err;
 
+     *address = shdr.sh_addr;
+     *offset = shdr.sh_offset;
+     ret = 0;
+out_err:
      elf_end(elf);
+     return ret;
+}
+
+#ifndef NO_LIBUNWIND_DEBUG_FRAME
+static u64 elf_section_offset(int fd, const char *name)
+{
+     u64 address, offset;
+
+     if (elf_section_address_and_offset(fd, name, &address, &offset))
+             return 0;
+
      return offset;
 }
+#endif
+
+static u64 elf_base_address(int fd)
+{
+     Elf *elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
+     GElf_Phdr phdr;
+     u64 retval = 0;
+     size_t i, phdrnum = 0;
+
+     if (elf == NULL)
+             return 0;
+     (void)elf_getphdrnum(elf, &phdrnum);
+     /* PT_LOAD segments are sorted by p_vaddr, so the first has the minimum p_vaddr. */
+     for (i = 0; i < phdrnum; i++) {
+             if (gelf_getphdr(elf, i, &phdr) && phdr.p_type == PT_LOAD) {
+                     retval = phdr.p_vaddr & -getpagesize();
+                     break;
+             }
+     }
+
+     elf_end(elf);
+     return retval;
+}
 
 #ifndef NO_LIBUNWIND_DEBUG_FRAME
 static int elf_is_exec(int fd, const char *name)

@@ -248,8 +282,7 @@ struct eh_frame_hdr {
 } __packed;
 
 static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
-                            u64 offset, u64 *table_data, u64 *segbase,
-                            u64 *fde_count)
+                            u64 offset, u64 *table_data_offset, u64 *fde_count)
 {
      struct eh_frame_hdr hdr;
      u8 *enc = (u8 *) &hdr.enc;

@@ -265,35 +298,47 @@ static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
      dw_read_encoded_value(enc, end, hdr.eh_frame_ptr_enc);
 
      *fde_count  = dw_read_encoded_value(enc, end, hdr.fde_count_enc);
-     *segbase    = offset;
-     *table_data = (enc - (u8 *) &hdr) + offset;
+     *table_data_offset = enc - (u8 *) &hdr;
      return 0;
 }
 
-static int read_unwind_spec_eh_frame(struct dso *dso, struct machine *machine,
+static int read_unwind_spec_eh_frame(struct dso *dso, struct unwind_info *ui,
                                   u64 *table_data, u64 *segbase,
                                   u64 *fde_count)
 {
-     int ret = -EINVAL, fd;
-     u64 offset = dso->data.eh_frame_hdr_offset;
+     struct map *map;
+     u64 base_addr = UINT64_MAX;
+     int ret, fd;
 
-     if (offset == 0) {
-             fd = dso__data_get_fd(dso, machine);
+     if (dso->data.eh_frame_hdr_offset == 0) {
+             fd = dso__data_get_fd(dso, ui->machine);
              if (fd < 0)
                      return -EINVAL;
 
              /* Check the .eh_frame section for unwinding info */
-             offset = elf_section_offset(fd, ".eh_frame_hdr");
-             dso->data.eh_frame_hdr_offset = offset;
+             ret = elf_section_address_and_offset(fd, ".eh_frame_hdr",
+                                                  &dso->data.eh_frame_hdr_addr,
+                                                  &dso->data.eh_frame_hdr_offset);
+             dso->data.elf_base_addr = elf_base_address(fd);
              dso__data_put_fd(dso);
+             if (ret || dso->data.eh_frame_hdr_offset == 0)
+                     return -EINVAL;
      }
 
-     if (offset)
-             ret = unwind_spec_ehframe(dso, machine, offset,
-                                       table_data, segbase,
-                                       fde_count);
-
-     return ret;
+     maps__for_each_entry(ui->thread->maps, map) {
+             if (map->dso == dso && map->start < base_addr)
+                     base_addr = map->start;
+     }
+     base_addr -= dso->data.elf_base_addr;
+     /* Address of .eh_frame_hdr */
+     *segbase = base_addr + dso->data.eh_frame_hdr_addr;
+     ret = unwind_spec_ehframe(dso, ui->machine, dso->data.eh_frame_hdr_offset,
+                                table_data, fde_count);
+     if (ret)
+             return ret;
+     /* binary_search_table offset plus .eh_frame_hdr address */
+     *table_data += *segbase;
+     return 0;
 }
 
 #ifndef NO_LIBUNWIND_DEBUG_FRAME

@@ -388,14 +433,14 @@ find_proc_info(unw_addr_space_t as, unw_word_t ip, unw_proc_info_t *pi,
      pr_debug("unwind: find_proc_info dso %s\n", map->dso->name);
 
      /* Check the .eh_frame section for unwinding info */
-     if (!read_unwind_spec_eh_frame(map->dso, ui->machine,
+     if (!read_unwind_spec_eh_frame(map->dso, ui,
                                     &table_data, &segbase, &fde_count)) {
              memset(&di, 0, sizeof(di));
              di.format   = UNW_INFO_FORMAT_REMOTE_TABLE;
              di.start_ip = map->start;
              di.end_ip   = map->end;
-             di.u.rti.segbase    = map->start + segbase - map->pgoff;
-             di.u.rti.table_data = map->start + table_data - map->pgoff;
+             di.u.rti.segbase    = segbase;
+             di.u.rti.table_data = table_data;
              di.u.rti.table_len  = fde_count * sizeof(struct table_entry)
                                    / sizeof(unw_word_t);
              ret = dwarf_search_unwind_table(as, ip, &di, pi,
--

2.36.1.124.g0e6072fb45-goog