kbuild: Avoid weak external linkage where possible

From: Ard Biesheuvel <[email protected]>

kallsyms is a directory of all the symbols in the vmlinux binary, and so creating it is somewhat of a chicken-and-egg problem, as its non-zero size affects the layout of the binary, and therefore the values of the symbols.

For this reason, the kernel is linked more than once, and the first pass does not include any kallsyms data at all. For the linker to accept this, the symbol declarations describing the kallsyms metadata are emitted as having weak linkage, so they can remain unsatisfied. During the subsequent passes, the weak references are satisfied by the kallsyms metadata that was constructed based on information gathered from the preceding passes.

Weak references lead to somewhat worse codegen, because taking their address may need to produce NULL (if the reference was unsatisfied), and this is not usually supported by RIP or PC relative symbol references.

Given that these references are ultimately always satisfied in the final link, let's drop the weak annotation, and instead, provide fallback definitions in the linker script that are only emitted if an unsatisfied reference exists.

While at it, drop the FRV specific annotation that these symbols reside in .rodata - FRV is long gone.

Tested-by Nick Desaulniers <[email protected]> # Boot

Reviewed-by Nick Desaulniers <[email protected]>

Reviewed-by Kees Cook <[email protected]>

Link https://lkml.kernel.org/r/20230504174320.3930345-1-ardb%40kernel.org open here

Signed-off-by Ard Biesheuvel <[email protected]>

From: Ard Biesheuvel <[email protected]>

Weak references are references that are permitted to remain unsatisfied in the final link. This means they cannot be implemented using place relative relocations, resulting in GOT entries when using position independent code generation.

The notes section should always exist, so the weak annotations can be omitted.

Signed-off-by Ard Biesheuvel <[email protected]>

From: Ard Biesheuvel <[email protected]>

If the BTF code is enabled in the build configuration, the start/stop BTF markers are guaranteed to exist in the final link but not during the first linker pass.

Avoid GOT based relocations to these markers in the final executable by providing preliminary definitions that will be used by the first linker pass, and superseded by the actual definitions in the subsequent ones.

Make the preliminary definitions dependent on CONFIG_DEBUG_INFO_BTF so that inadvertent references to this section will trigger a link failure if they occur in code that does not honour CONFIG_DEBUG_INFO_BTF.

Note that Clang will notice that taking the address of__start_BTF cannot yield NULL any longer, so testing for that condition is no longer needed.

Acked-by Andrii Nakryiko <[email protected]>

Signed-off-by Ard Biesheuvel <[email protected]>

From: Ard Biesheuvel <[email protected]>

If the BTF code is enabled in the build configuration, the start/stop BTF markers are guaranteed to exist in the final link but not during the first linker pass.

Avoid GOT based relocations to these markers in the final executable by providing preliminary definitions that will be used by the first linker pass, and superseded by the actual definitions in the subsequent ones.

Make the preliminary definitions dependent on CONFIG_DEBUG_INFO_BTF so that inadvertent references to this section will trigger a link failure if they occur in code that does not honour CONFIG_DEBUG_INFO_BTF.

Note that Clang will notice that taking the address of__start_BTF cannot yield NULL any longer, so testing for that condition is no longer needed.

Acked-by Andrii Nakryiko <[email protected]>

Signed-off-by Ard Biesheuvel <[email protected]>

---
 include/asm-generic/vmlinux.lds.h | 9 +++++++++
 kernel/bpf/btf.c                  | 4 ++--
 kernel/bpf/sysfs_btf.c            | 6 +++---
 3 files changed, 14 insertions(+), 5 deletions(-)

include/asm-generic/vmlinux.lds.h +1 -0

index e8449be62058..4cb3d88449e5 100644
@@ -456,6 +456,7 @@
  * independent code.
  */
 #define PRELIMINARY_SYMBOL_DEFINITIONS					\
+	PRELIMINARY_BTF_DEFINITIONS					\
 	PROVIDE(kallsyms_addresses = .);				\
 	PROVIDE(kallsyms_offsets = .);					\
 	PROVIDE(kallsyms_names = .);					\

include/asm-generic/vmlinux.lds.h +1 -0

+
 /*
  * Read only Data
  */

kernel/bpf/btf.c +2 -2

index 90c4a32d89ff..46a56bf067a8 100644
@@ -5642,8 +5642,8 @@ static struct btf *btf_parse(const union bpf_attr *attr, bpfptr_t uattr, u32 uat
 	return ERR_PTR(err);
 }
 
-extern char __weak __start_BTF[];
-extern char __weak __stop_BTF[];
+extern char __start_BTF[];
+extern char __stop_BTF[];
 extern struct btf *btf_vmlinux;
 
 #define BPF_MAP_TYPE(_id, _ops)

kernel/bpf/sysfs_btf.c +3 -3

index ef6911aee3bb..fedb54c94cdb 100644
@@ -9,8 +9,8 @@
 #include <linux/sysfs.h>
 
 /* See scripts/link-vmlinux.sh, gen_btf() func for details */
-extern char __weak __start_BTF[];
-extern char __weak __stop_BTF[];
+extern char __start_BTF[];
+extern char __stop_BTF[];
 
 static ssize_t
 btf_vmlinux_read(struct file *file, struct kobject *kobj,

@@ -32,7 +32,7 @@ static int __init btf_vmlinux_init(void)
 {
 	bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
 
-	if (!__start_BTF || bin_attr_btf_vmlinux.size == 0)
+	if (bin_attr_btf_vmlinux.size == 0)
 		return 0;
 
 	btf_kobj = kobject_create_and_add("btf", kernel_kobj);

Signature · 1 line

2.44.0.478.gd926399ef9-goog

Quoted 39 lines · Ard Biesheuvel

From: Ard Biesheuvel <[email protected]>

If the BTF code is enabled in the build configuration, the start/stop BTF markers are guaranteed to exist in the final link but not during the first linker pass.

Avoid GOT based relocations to these markers in the final executable by providing preliminary definitions that will be used by the first linker pass, and superseded by the actual definitions in the subsequent ones.

Make the preliminary definitions dependent on CONFIG_DEBUG_INFO_BTF so that inadvertent references to this section will trigger a link failure if they occur in code that does not honour CONFIG_DEBUG_INFO_BTF.

Note that Clang will notice that taking the address of__start_BTF cannot yield NULL any longer, so testing for that condition is no longer needed.

Acked-by Andrii Nakryiko <[email protected]>

Signed-off-by Ard Biesheuvel <[email protected]>

---
 include/asm-generic/vmlinux.lds.h | 9 +++++++++
 kernel/bpf/btf.c                  | 4 ++--
 kernel/bpf/sysfs_btf.c            | 6 +++---
 3 files changed, 14 insertions(+), 5 deletions(-)

include/asm-generic/vmlinux.lds.h +1 -0

index e8449be62058..4cb3d88449e5 100644
@@ -456,6 +456,7 @@
  * independent code.
  */
 #define PRELIMINARY_SYMBOL_DEFINITIONS                                       \
+     PRELIMINARY_BTF_DEFINITIONS                                     \
      PROVIDE(kallsyms_addresses = .);                                \
      PROVIDE(kallsyms_offsets = .);                                  \
      PROVIDE(kallsyms_names = .);                                    \

include/asm-generic/vmlinux.lds.h line 476

Quoted 10 lines

include/asm-generic/vmlinux.lds.h +7 -0

@@ -466,6 +467,14 @@
      PROVIDE(kallsyms_markers = .);                                  \
      PROVIDE(kallsyms_seqs_of_names = .);
 
+#ifdef CONFIG_DEBUG_INFO_BTF
+#define PRELIMINARY_BTF_DEFINITIONS                                  \
+     PROVIDE(__start_BTF = .);                                       \
+     PROVIDE(__stop_BTF = .);
+#else
+#define PRELIMINARY_BTF_DEFINITIONS
+#endif

hi,
I'm getting following compilation fail when CONFIG_DEBUG_INFO_BTF is disabled

[jolsa@krava linux-qemu]$ make
CALL scripts/checksyscalls.sh
DESCEND objtool
INSTALL libsubcmd_headers
UPD include/generated/utsversion.h
CC init/version-timestamp.o
LD .tmp_vmlinux.kallsyms1
ld: kernel/bpf/btf.o: in function `btf_parse_vmlinux':
/home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5988: undefined reference to `__start_BTF'
ld: /home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5989: undefined reference to `__stop_BTF'
ld: /home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5989: undefined reference to `__start_BTF' make[2]: *** [scripts/Makefile.vmlinux:37: vmlinux] Error 1
make[1]: *** [/home/jolsa/kernel/linux-qemu/Makefile:1160: vmlinux] Error 2
make: *** [Makefile:240: __sub-make] Error 2

maybe the assumption was that kernel/bpf/btf.o is compiled only for CONFIG_DEBUG_INFO_BTF, but it's actually:

  obj-$(CONFIG_BPF_SYSCALL) += btf.o memalloc.o

I guess we just need !CONFIG_DEBUG_INFO_BTF version of btf_parse_vmlinux function

Quoted 70 lines · Jiri Olsa

Quoted 39 lines · Ard Biesheuvel

From: Ard Biesheuvel <[email protected]>

If the BTF code is enabled in the build configuration, the start/stop BTF markers are guaranteed to exist in the final link but not during the first linker pass.

Avoid GOT based relocations to these markers in the final executable by providing preliminary definitions that will be used by the first linker pass, and superseded by the actual definitions in the subsequent ones.

Make the preliminary definitions dependent on CONFIG_DEBUG_INFO_BTF so that inadvertent references to this section will trigger a link failure if they occur in code that does not honour CONFIG_DEBUG_INFO_BTF.

Note that Clang will notice that taking the address of__start_BTF cannot yield NULL any longer, so testing for that condition is no longer needed.

Acked-by Andrii Nakryiko <[email protected]>

Signed-off-by Ard Biesheuvel <[email protected]>

---
 include/asm-generic/vmlinux.lds.h | 9 +++++++++
 kernel/bpf/btf.c                  | 4 ++--
 kernel/bpf/sysfs_btf.c            | 6 +++---
 3 files changed, 14 insertions(+), 5 deletions(-)

include/asm-generic/vmlinux.lds.h +1 -0

index e8449be62058..4cb3d88449e5 100644
@@ -456,6 +456,7 @@
  * independent code.
  */
 #define PRELIMINARY_SYMBOL_DEFINITIONS                                       \
+     PRELIMINARY_BTF_DEFINITIONS                                     \
      PROVIDE(kallsyms_addresses = .);                                \
      PROVIDE(kallsyms_offsets = .);                                  \
      PROVIDE(kallsyms_names = .);                                    \

include/asm-generic/vmlinux.lds.h line 476

Quoted 10 lines

include/asm-generic/vmlinux.lds.h +7 -0

@@ -466,6 +467,14 @@
      PROVIDE(kallsyms_markers = .);                                  \
      PROVIDE(kallsyms_seqs_of_names = .);
 
+#ifdef CONFIG_DEBUG_INFO_BTF
+#define PRELIMINARY_BTF_DEFINITIONS                                  \
+     PROVIDE(__start_BTF = .);                                       \
+     PROVIDE(__stop_BTF = .);
+#else
+#define PRELIMINARY_BTF_DEFINITIONS
+#endif

hi,
I'm getting following compilation fail when CONFIG_DEBUG_INFO_BTF is disabled

[jolsa@krava linux-qemu]$ make
CALL scripts/checksyscalls.sh
DESCEND objtool
INSTALL libsubcmd_headers
UPD include/generated/utsversion.h
CC init/version-timestamp.o
LD .tmp_vmlinux.kallsyms1
ld: kernel/bpf/btf.o: in function `btf_parse_vmlinux':
/home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5988: undefined reference to `__start_BTF'
ld: /home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5989: undefined reference to `__stop_BTF'
ld: /home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5989: undefined reference to `__start_BTF' make[2]: *** [scripts/Makefile.vmlinux:37: vmlinux] Error 1
make[1]: *** [/home/jolsa/kernel/linux-qemu/Makefile:1160: vmlinux] Error 2
make: *** [Makefile:240: __sub-make] Error 2

Quoted 5 lines

maybe the assumption was that kernel/bpf/btf.o is compiled only for CONFIG_DEBUG_INFO_BTF, but it's actually:

  obj-$(CONFIG_BPF_SYSCALL) += btf.o memalloc.o

Interesting. I did build test this with and without CONFIG_DEBUG_INFO_BTF, but not with CONFIG_BPF_SYSCALL=y and CONFIG_DEBUG_INFO_BTF=n.

Quoted 3 lines

I guess we just need !CONFIG_DEBUG_INFO_BTF version of btf_parse_vmlinux function

The below gives me a working build.

kernel/bpf/btf.c +1 -0

+
        env = kzalloc(sizeof(*env), GFP_KERNEL | __GFP_NOWARN);
        if (!env)
                return ERR_PTR(-ENOMEM);