Fix CONFIG_FUNCTION_TRACER with clang

Clang can generate R_RISCV_CALL_PLT relocations to _mcount:

$ llvm-objdump -dr build/riscv/init/main.o | rg mcount
               000000000000000e:  R_RISCV_CALL_PLT     _mcount
               000000000000004e:  R_RISCV_CALL_PLT     _mcount

After this, the __start_mcount_loc section is properly generated and function tracing still works.

Cc [email protected]

Link https://github.com/ClangBuiltLinux/linux/issues/1331

Signed-off-by Nathan Chancellor <[email protected]>

---
scripts/recordmcount.pl | 2 +-

1 file changed, 1 insertion(+), 1 deletion(-)

scripts/recordmcount.pl +1 -1

index 867860ea57da..a36df04cfa09 100755
@@ -392,7 +392,7 @@ if ($arch eq "x86_64") {
    $mcount_regex = "^\\s*([0-9a-fA-F]+):.*\\s_mcount\$";
 } elsif ($arch eq "riscv") {
    $function_regex = "^([0-9a-fA-F]+)\\s+<([^.0-9][0-9a-zA-Z_\\.]+)>:";
-    $mcount_regex = "^\\s*([0-9a-fA-F]+):\\sR_RISCV_CALL\\s_mcount\$";
+    $mcount_regex = "^\\s*([0-9a-fA-F]+):\\sR_RISCV_CALL(_PLT)?\\s_mcount\$";
    $type = ".quad";
    $alignment = 2;
 } elsif ($arch eq "nds32") {

Signature · 6 lines

2.31.0

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Prior to clang 13.0.0, the RISC-V name for the mcount symbol was "mcount", which differs from the GCC version of "_mcount", which results in the following errors:

riscv64-linux-gnu-ld: init/main.o: in function `__traceiter_initcall_level':
main.c:(.text+0xe): undefined reference to `mcount'
riscv64-linux-gnu-ld: init/main.o: in function `__traceiter_initcall_start':
main.c:(.text+0x4e): undefined reference to `mcount'
riscv64-linux-gnu-ld: init/main.o: in function `__traceiter_initcall_finish':
main.c:(.text+0x92): undefined reference to `mcount'
riscv64-linux-gnu-ld: init/main.o: in function `.LBB32_28':
main.c:(.text+0x30c): undefined reference to `mcount'
riscv64-linux-gnu-ld: init/main.o: in function `free_initmem':
main.c:(.text+0x54c): undefined reference to `mcount'

This has been corrected in https://reviews.llvm.org/D98881 but the minimum supported clang version is 10.0.1. To avoid build errors and to gain a working function tracer, adjust the name of the mcount symbol for older versions of clang in mount.S and recordmcount.pl.

Cc [email protected]

Link https://github.com/ClangBuiltLinux/linux/issues/1331

Signed-off-by Nathan Chancellor <[email protected]>

---
 arch/riscv/include/asm/ftrace.h | 14 ++++++++++++--
 arch/riscv/kernel/mcount.S      | 10 +++++-----
 scripts/recordmcount.pl         |  2 +-
 3 files changed, 18 insertions(+), 8 deletions(-)

arch/riscv/include/asm/ftrace.h +12 -2

index 845002cc2e57..04dad3380041 100644
@@ -13,9 +13,19 @@
 #endif
 #define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 
+/*
+ * Clang prior to 13 had "mcount" instead of "_mcount":
+ * https://reviews.llvm.org/D98881
+ */
+#if defined(CONFIG_CC_IS_GCC) || CONFIG_CLANG_VERSION >= 130000
+#define MCOUNT_NAME _mcount
+#else
+#define MCOUNT_NAME mcount
+#endif
+
 #define ARCH_SUPPORTS_FTRACE_OPS 1
 #ifndef __ASSEMBLY__
-void _mcount(void);
+void MCOUNT_NAME(void);
 static inline unsigned long ftrace_call_adjust(unsigned long addr)
 {
        return addr;

@@ -36,7 +46,7 @@ struct dyn_arch_ftrace {
  * both auipc and jalr at the same time.
  */
 
-#define MCOUNT_ADDR            ((unsigned long)_mcount)
+#define MCOUNT_ADDR            ((unsigned long)MCOUNT_NAME)
 #define JALR_SIGN_MASK         (0x00000800)
 #define JALR_OFFSET_MASK       (0x00000fff)
 #define AUIPC_OFFSET_MASK      (0xfffff000)

arch/riscv/kernel/mcount.S +5 -5

index 8a5593ff9ff3..6d462681c9c0 100644
@@ -47,8 +47,8 @@
 
 ENTRY(ftrace_stub)
 #ifdef CONFIG_DYNAMIC_FTRACE
-       .global _mcount
-       .set    _mcount, ftrace_stub
+       .global MCOUNT_NAME
+       .set    MCOUNT_NAME, ftrace_stub
 #endif
        ret
 ENDPROC(ftrace_stub)

@@ -78,7 +78,7 @@ ENDPROC(return_to_handler)
 #endif
 
 #ifndef CONFIG_DYNAMIC_FTRACE
-ENTRY(_mcount)
+ENTRY(MCOUNT_NAME)
        la      t4, ftrace_stub
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
        la      t0, ftrace_graph_return

@@ -124,6 +124,6 @@ do_trace:
        jalr    t5
        RESTORE_ABI_STATE
        ret
-ENDPROC(_mcount)
+ENDPROC(MCOUNT_NAME)
 #endif
-EXPORT_SYMBOL(_mcount)
+EXPORT_SYMBOL(MCOUNT_NAME)

scripts/recordmcount.pl +1 -2

index a36df04cfa09..7b83a1aaec98 100755
@@ -392,7 +392,7 @@ if ($arch eq "x86_64") {
     $mcount_regex = "^\\s*([0-9a-fA-F]+):.*\\s_mcount\$";
 } elsif ($arch eq "riscv") {
     $function_regex = "^([0-9a-fA-F]+)\\s+<([^.0-9][0-9a-zA-Z_\\.]+)>:";
-    $mcount_regex = "^\\s*([0-9a-fA-F]+):\\sR_RISCV_CALL(_PLT)?\\s_mcount\$";
+    $mcount_regex = "^\\s*([0-9a-fA-F]+):\\sR_RISCV_CALL(_PLT)?\\s_?mcount\$";
     $type = ".quad";
     $alignment = 2;
 } elsif ($arch eq "nds32") {
--

2.31.0

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clang prior to 13.0.0 does not support -fpatchable-function-entry for RISC-V.

clang: error: unsupported option '-fpatchable-function-entry=8' for target 'riscv64-unknown-linux-gnu'

To avoid this error, only select HAVE_DYNAMIC_FTRACE when this option is not available.

Fixes afc76b8b8011 ("riscv: Using PATCHABLE_FUNCTION_ENTRY instead of MCOUNT")

Link https://github.com/ClangBuiltLinux/linux/issues/1268

Reported-by kernel test robot <[email protected]>

Signed-off-by Nathan Chancellor <[email protected]>

---
arch/riscv/Kconfig | 2 +-

1 file changed, 1 insertion(+), 1 deletion(-)

arch/riscv/Kconfig +1 -1

index 87d7b52f278f..ba1d07640b66 100644
@@ -227,7 +227,7 @@ config ARCH_RV64I
 	bool "RV64I"
 	select 64BIT
 	select ARCH_SUPPORTS_INT128 if CC_HAS_INT128 && GCC_VERSION >= 50000
-	select HAVE_DYNAMIC_FTRACE if MMU
+	select HAVE_DYNAMIC_FTRACE if MMU && $(cc-option,-fpatchable-function-entry=8)
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_FUNCTION_GRAPH_TRACER

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2.31.0

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Hi all,

This series fixes function tracing with clang.

Patch 1 adjusts the mcount regex in scripts/recordmcount.pl to handle the presence of PLT relocations, which happen with clang. Without this, the mcount_loc section will not be created properly.

Patch 2 adds a workaround for clang less than 13.0.0 in relation to the mcount symbol name, which was "mcount" rather than "_mcount". This was written as a separate patch so that it can be reverted when the minimum clang version is bumped to clang 13.0.0.

Patch 3 avoids a build error when -fpatchable-function-entry is not available, which is the case with clang less than 13.0.0. This will make dynamic ftrace unavailable but all of the other function tracing should work due to the prescence of the previous patch.

I am hoping this series can go in as fixes for 5.12, due to patch 3, but if not, they can always be backported (patches 1 and 2 are already marked for stable).

This series has been build tested with gcc-8 through gcc-10 and clang-11 through clang-13 with defconfig and nommu_virt_defconfig plus CONFIG_FTRACE=y and CONFIG_FUNCTION_TRACER=y then boot tested under QEMU.

Cheers,
Nathan

Nathan Chancellor (3):
scripts/recordmcount.pl: Fix RISC-V regex for clang
riscv: Workaround mcount name prior to clang-13
riscv: Select HAVE_DYNAMIC_FTRACE when -fpatchable-function-entry is
available

 arch/riscv/Kconfig              |  2 +-
 arch/riscv/include/asm/ftrace.h | 14 ++++++++++++--
 arch/riscv/kernel/mcount.S      | 10 +++++-----
 scripts/recordmcount.pl         |  2 +-
 4 files changed, 19 insertions(+), 9 deletions(-)

--
2.31.0

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Quoted 31 lines · Nathan Chancellor

Hi all,

This series fixes function tracing with clang.

Patch 1 adjusts the mcount regex in scripts/recordmcount.pl to handle the presence of PLT relocations, which happen with clang. Without this, the mcount_loc section will not be created properly.

Patch 2 adds a workaround for clang less than 13.0.0 in relation to the mcount symbol name, which was "mcount" rather than "_mcount". This was written as a separate patch so that it can be reverted when the minimum clang version is bumped to clang 13.0.0.

Patch 3 avoids a build error when -fpatchable-function-entry is not available, which is the case with clang less than 13.0.0. This will make dynamic ftrace unavailable but all of the other function tracing should work due to the prescence of the previous patch.

I am hoping this series can go in as fixes for 5.12, due to patch 3, but if not, they can always be backported (patches 1 and 2 are already marked for stable).

This series has been build tested with gcc-8 through gcc-10 and clang-11 through clang-13 with defconfig and nommu_virt_defconfig plus CONFIG_FTRACE=y and CONFIG_FUNCTION_TRACER=y then boot tested under QEMU.

Cheers,
Nathan

Quoted 6 lines

Nathan Chancellor (3):
scripts/recordmcount.pl: Fix RISC-V regex for clang
riscv: Workaround mcount name prior to clang-13
riscv: Select HAVE_DYNAMIC_FTRACE when -fpatchable-function-entry is
available

Does this only fixes stuff for clang + riscv?

If so, please put a label "riscv" also in the cover-letter.

- Sedat -

Quoted 9 lines

 arch/riscv/Kconfig              |  2 +-
 arch/riscv/include/asm/ftrace.h | 14 ++++++++++++--
 arch/riscv/kernel/mcount.S      | 10 +++++-----
 scripts/recordmcount.pl         |  2 +-
 4 files changed, 19 insertions(+), 9 deletions(-)

--
2.31.0

Quoted 32 lines · Sedat Dilek

Quoted 31 lines · Nathan Chancellor

Hi all,

This series fixes function tracing with clang.

Patch 1 adjusts the mcount regex in scripts/recordmcount.pl to handle the presence of PLT relocations, which happen with clang. Without this, the mcount_loc section will not be created properly.

Patch 2 adds a workaround for clang less than 13.0.0 in relation to the mcount symbol name, which was "mcount" rather than "_mcount". This was written as a separate patch so that it can be reverted when the minimum clang version is bumped to clang 13.0.0.

Patch 3 avoids a build error when -fpatchable-function-entry is not available, which is the case with clang less than 13.0.0. This will make dynamic ftrace unavailable but all of the other function tracing should work due to the prescence of the previous patch.

I am hoping this series can go in as fixes for 5.12, due to patch 3, but if not, they can always be backported (patches 1 and 2 are already marked for stable).

This series has been build tested with gcc-8 through gcc-10 and clang-11 through clang-13 with defconfig and nommu_virt_defconfig plus CONFIG_FTRACE=y and CONFIG_FUNCTION_TRACER=y then boot tested under QEMU.

Cheers,
Nathan

Quoted 8 lines

Quoted 6 lines

Nathan Chancellor (3):
scripts/recordmcount.pl: Fix RISC-V regex for clang
riscv: Workaround mcount name prior to clang-13
riscv: Select HAVE_DYNAMIC_FTRACE when -fpatchable-function-entry is
available

Does this only fixes stuff for clang + riscv?

Yes.

Quoted 1 line

If so, please put a label "riscv" also in the cover-letter.

Sure, my apologies for not doing that in the first place, I must have been in a rush with the cover letter.

In my defense, I think the titles of my commit messages and the diffstat below make that obvious without the tag :)

Quoted 12 lines

- Sedat -

Quoted 9 lines

 arch/riscv/Kconfig              |  2 +-
 arch/riscv/include/asm/ftrace.h | 14 ++++++++++++--
 arch/riscv/kernel/mcount.S      | 10 +++++-----
 scripts/recordmcount.pl         |  2 +-
 4 files changed, 19 insertions(+), 9 deletions(-)

--
2.31.0

Hi all,

This series fixes function tracing with clang.

Patch 1 adjusts the mcount regex in scripts/recordmcount.pl to handle the presence of PLT relocations, which happen with clang. Without this, the mcount_loc section will not be created properly.

Patch 2 adds a workaround for clang less than 13.0.0 in relation to the mcount symbol name, which was "mcount" rather than "_mcount". This was written as a separate patch so that it can be reverted when the minimum clang version is bumped to clang 13.0.0.

Patch 3 avoids a build error when -fpatchable-function-entry is not available, which is the case with clang less than 13.0.0. This will make dynamic ftrace unavailable but all of the other function tracing should work due to the prescence of the previous patch.

I am hoping this series can go in as fixes for 5.12, due to patch 3, but if not, they can always be backported (patches 1 and 2 are already marked for stable).

This series has been build tested with gcc-8 through gcc-10 and clang-11 through clang-13 with defconfig and nommu_virt_defconfig plus CONFIG_FTRACE=y and CONFIG_FUNCTION_TRACER=y then boot tested under QEMU.

Cheers,
Nathan

Nathan Chancellor (3):
scripts/recordmcount.pl: Fix RISC-V regex for clang
riscv: Workaround mcount name prior to clang-13
riscv: Select HAVE_DYNAMIC_FTRACE when -fpatchable-function-entry is
available

 arch/riscv/Kconfig              |  2 +-
 arch/riscv/include/asm/ftrace.h | 14 ++++++++++++--
 arch/riscv/kernel/mcount.S      | 10 +++++-----
 scripts/recordmcount.pl         |  2 +-
 4 files changed, 19 insertions(+), 9 deletions(-)