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kallsyms: ignore local block labels generated by compiler

PATCH In review Changbin Du 2 participants 4 messages Jan 28, 2022 – Jan 29, 2022 RISC-V

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Changbin Du

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#0 lore raw

PATCH 1/1 kallsyms: ignore local block labels generated by compiler

The llvm compiler can generate lots of local block labels and they might overlap with C defined symbols. So let's ignore such local labels.

Before this change, dumpstack shows a local symbol for epc:

[    0.040341][    T0] Hardware name: riscv-virtio,qemu (DT)
[    0.040376][    T0] epc : .LBB6_14+0x22/0x6a
[    0.040452][    T0]  ra : restore_all+0x12/0x6e

After this change, the C defined symbol is shown which is expected:

[    0.035795][    T0] Hardware name: riscv-virtio,qemu (DT)
[    0.036332][    T0] epc : trace_hardirqs_on+0x54/0x13c
[    0.036567][    T0]  ra : restore_all+0x12/0x6e
Signed-off-by Changbin Du <[email protected]>
---
 scripts/kallsyms.c | 1 +
 1 file changed, 1 insertion(+)
scripts/kallsyms.c +1 -0
index 54ad86d13784..5f4be9d72a32 100644
@@ -108,6 +108,7 @@ static bool is_ignored_symbol(const char *name, char type)
 	/* Symbol names that begin with the following are ignored.*/
 	static const char * const ignored_prefixes[] = {
 		"$",			/* local symbols for ARM, MIPS, etc. */
+		".LBB",			/* local block labels generated by compiler */
 		".LASANPC",		/* s390 kasan local symbols */
 		"__crc_",		/* modversions */
 		"__efistub_",		/* arm64 EFI stub namespace */
Signature · 1 line
2.32.0
author
#1 lore raw
Quoted 23 lines · Changbin Du

The llvm compiler can generate lots of local block labels and they might overlap with C defined symbols. So let's ignore such local labels.

Before this change, dumpstack shows a local symbol for epc:

[    0.040341][    T0] Hardware name: riscv-virtio,qemu (DT)
[    0.040376][    T0] epc : .LBB6_14+0x22/0x6a
[    0.040452][    T0]  ra : restore_all+0x12/0x6e

After this change, the C defined symbol is shown which is expected:

[    0.035795][    T0] Hardware name: riscv-virtio,qemu (DT)
[    0.036332][    T0] epc : trace_hardirqs_on+0x54/0x13c
[    0.036567][    T0]  ra : restore_all+0x12/0x6e
Signed-off-by Changbin Du <[email protected]>
---
 scripts/kallsyms.c | 1 +
 1 file changed, 1 insertion(+)
scripts/kallsyms.c +0 -0
index 54ad86d13784..5f4be9d72a32 100644

scripts/kallsyms.c static bool is_ignored_symbol(const char *name, char type) · line 111

Quoted 4 lines
scripts/kallsyms.c +1 -0
@@ -108,6 +108,7 @@ static bool is_ignored_symbol(const char *name, char type)
 	/* Symbol names that begin with the following are ignored.*/
 	static const char * const ignored_prefixes[] = {
 		"$",			/* local symbols for ARM, MIPS, etc. */
+		".LBB",			/* local block labels generated by compiler */

I aslo found many symbols like '.Ltmpxxx', '.L__unnamed_xx'. So should we just ignore all symbols prefixed by '.L'?

Quoted 6 lines
scripts/kallsyms.c +0 -0
 		".LASANPC",		/* s390 kasan local symbols */
 		"__crc_",		/* modversions */
 		"__efistub_",		/* arm64 EFI stub namespace */
Signature · 1 line
2.32.0
Signature · 2 lines
Cheers,
Changbin Du
#2 lore raw
Quoted 20 lines · Changbin Du
Quoted 18 lines · Changbin Du

The llvm compiler can generate lots of local block labels and they might overlap with C defined symbols. So let's ignore such local labels.

Before this change, dumpstack shows a local symbol for epc:

[    0.040341][    T0] Hardware name: riscv-virtio,qemu (DT)
[    0.040376][    T0] epc : .LBB6_14+0x22/0x6a
[    0.040452][    T0]  ra : restore_all+0x12/0x6e

After this change, the C defined symbol is shown which is expected:

[    0.035795][    T0] Hardware name: riscv-virtio,qemu (DT)
[    0.036332][    T0] epc : trace_hardirqs_on+0x54/0x13c
[    0.036567][    T0]  ra : restore_all+0x12/0x6e
Signed-off-by Changbin Du <[email protected]>
---
 scripts/kallsyms.c | 1 +
 1 file changed, 1 insertion(+)
Quoted 11 lines

scripts/kallsyms.c static bool is_ignored_symbol(const char *name, char type) · line 111

Quoted 9 lines
scripts/kallsyms.c +1 -0
index 54ad86d13784..5f4be9d72a32 100644
@@ -108,6 +108,7 @@ static bool is_ignored_symbol(const char *name, char type)
      /* Symbol names that begin with the following are ignored.*/
      static const char * const ignored_prefixes[] = {
              "$",                    /* local symbols for ARM, MIPS, etc. */
+             ".LBB",                 /* local block labels generated by compiler */

I aslo found many symbols like '.Ltmpxxx', '.L__unnamed_xx'. So should we just ignore all symbols prefixed by '.L'?

.L prefixes are "local labels" meaning they don't get an entry in the symbol table. The trade off is that there is no such convention between compiler generated local label naming conventions, vs hand written ones. If we omit all local labels, then it might be surprising that a handwritten local label disappeared from this list. I think though that we should omit all local labels, and if someone needs a label to appear in their dumpstack, then they should simply drop the .L prefix on their handwritten label.

Quoted 11 lines
Quoted 7 lines
scripts/kallsyms.c +0 -1
              ".LASANPC",             /* s390 kasan local symbols */
              "__crc_",               /* modversions */
              "__efistub_",           /* arm64 EFI stub namespace */
--

2.32.0

--
Cheers,
Changbin Du

Signature · 2 lines
Thanks,
~Nick Desaulniers
author
#3 lore raw
Quoted 33 lines · Nick Desaulniers
Quoted 20 lines · Changbin Du
Quoted 18 lines · Changbin Du

The llvm compiler can generate lots of local block labels and they might overlap with C defined symbols. So let's ignore such local labels.

Before this change, dumpstack shows a local symbol for epc:

[    0.040341][    T0] Hardware name: riscv-virtio,qemu (DT)
[    0.040376][    T0] epc : .LBB6_14+0x22/0x6a
[    0.040452][    T0]  ra : restore_all+0x12/0x6e

After this change, the C defined symbol is shown which is expected:

[    0.035795][    T0] Hardware name: riscv-virtio,qemu (DT)
[    0.036332][    T0] epc : trace_hardirqs_on+0x54/0x13c
[    0.036567][    T0]  ra : restore_all+0x12/0x6e
Signed-off-by Changbin Du <[email protected]>
---
 scripts/kallsyms.c | 1 +
 1 file changed, 1 insertion(+)
Quoted 11 lines

scripts/kallsyms.c static bool is_ignored_symbol(const char *name, char type) · line 111

Quoted 9 lines
scripts/kallsyms.c +1 -0
index 54ad86d13784..5f4be9d72a32 100644
@@ -108,6 +108,7 @@ static bool is_ignored_symbol(const char *name, char type)
      /* Symbol names that begin with the following are ignored.*/
      static const char * const ignored_prefixes[] = {
              "$",                    /* local symbols for ARM, MIPS, etc. */
+             ".LBB",                 /* local block labels generated by compiler */

I aslo found many symbols like '.Ltmpxxx', '.L__unnamed_xx'. So should we just ignore all symbols prefixed by '.L'?

Quoted 9 lines

.L prefixes are "local labels" meaning they don't get an entry in the symbol table. The trade off is that there is no such convention between compiler generated local label naming conventions, vs hand written ones. If we omit all local labels, then it might be surprising that a handwritten local label disappeared from this list. I think though that we should omit all local labels, and if someone needs a label to appear in their dumpstack, then they should simply drop the .L prefix on their handwritten label.

Agree. I checked vmlinux for riscv and x86, seeing no handwritten function labels prefixed by '.L'. (just some data definition symbols). It should be safte to simply ignore all these symbols.

Quoted 17 lines
Quoted 11 lines
Quoted 7 lines
scripts/kallsyms.c +0 -1
              ".LASANPC",             /* s390 kasan local symbols */
              "__crc_",               /* modversions */
              "__efistub_",           /* arm64 EFI stub namespace */
--

2.32.0

--
Cheers,
Changbin Du

Signature · 2 lines
Thanks,
~Nick Desaulniers
Signature · 2 lines
Cheers,
Changbin Du