On 08/29/2023 02:45 PM, Xi Ruoyao wrote:
> On Tue, 2023-08-29 at 14:28 +0800, Tiezhu Yang wrote:
>> The initial aim is to silence the following objtool warnings:
>>
>> arch/loongarch/kernel/fpu.o: warning: objtool: _save_fp_context() falls through to next function fault()
>> arch/loongarch/kernel/fpu.o: warning: objtool: _restore_fp_context() falls through to next function fault()
>> arch/loongarch/kernel/fpu.o: warning: objtool: _save_lsx_context() falls through to next function fault()
>> arch/loongarch/kernel/fpu.o: warning: objtool: _restore_lsx_context() falls through to next function fault()
>> arch/loongarch/kernel/fpu.o: warning: objtool: _save_lasx_context() falls through to next function fault()
>> arch/loongarch/kernel/fpu.o: warning: objtool: _restore_lasx_context() falls through to next function fault()
>>
>> Obviously, the symbol fault is not a function, it is just a local label,
>
> Hmm why? To me this seems a function. We don't branch to it but store
> its address (a "function pointer") in the extable.
>
> And these warnings do not make any sense to me:
>
> /* snip */
>
>> diff --git a/arch/loongarch/kernel/fpu.S b/arch/loongarch/kernel/fpu.S
>> index b4032de..7defe50 100644
>> --- a/arch/loongarch/kernel/fpu.S
>> +++ b/arch/loongarch/kernel/fpu.S
>> @@ -521,7 +521,7 @@ SYM_FUNC_START(_restore_lasx_context)
>> jr ra
>
> _restore_lasx_context returns with this instruction. How can it fall
> through into fault?
>
>> SYM_FUNC_END(_restore_lasx_context)
>
>> -SYM_FUNC_START(fault)
>> +SYM_CODE_START_LOCAL(fault)
>> li.w a0, -EFAULT # failure
>> jr ra
>> -SYM_FUNC_END(fault)
>> +SYM_CODE_END(fault)
>
In the current code, SYM_FUNC_END() defines the symbol as SYM_T_FUNC
which is STT_FUNC, the objtool warnings are generated through the
following code.
arch/loongarch/include/asm/linkage.h
#define SYM_FUNC_END(name) \
.cfi_endproc; \
SYM_END(name, SYM_T_FUNC)
include/linux/linkage.h
/* SYM_T_FUNC -- type used by assembler to mark functions */
#ifndef SYM_T_FUNC
#define SYM_T_FUNC STT_FUNC
#endif
tools/objtool/include/objtool/check.h
static inline struct symbol *insn_func(struct instruction *insn)
{
struct symbol *sym = insn->sym;
if (sym && sym->type != STT_FUNC)
sym = NULL;
return sym;
}
tools/objtool/check.c
static int validate_branch(struct objtool_file *file, struct symbol *func,
struct instruction *insn, struct insn_state state)
{
...
while (1) {
next_insn = next_insn_to_validate(file, insn);
if (func && insn_func(insn) && func != insn_func(insn)->pfunc) {
/* Ignore KCFI type preambles, which always fall through */
if (!strncmp(func->name, "__cfi_", 6) ||
!strncmp(func->name, "__pfx_", 6))
return 0;
WARN("%s() falls through to next function %s()",
func->name, insn_func(insn)->name);
return 1;
}
...
}
We can see that the fixup can be a local label in the following code.
arch/loongarch/include/asm/asm-extable.h
#define __ASM_EXTABLE_RAW(insn, fixup, type, data) \
.pushsection __ex_table, "a"; \
.balign 4; \
.long ((insn) - .); \
.long ((fixup) - .); \
.short (type); \
.short (data); \
.popsection;
.macro _asm_extable, insn, fixup
__ASM_EXTABLE_RAW(\insn, \fixup, EX_TYPE_FIXUP, 0)
.endm
Like arch/loongarch/lib/*.S does, I prefer the following changes,
if you are ok, I will send v2 later.
diff --git a/arch/loongarch/kernel/fpu.S b/arch/loongarch/kernel/fpu.S
index b4032deb8e3b..3177725ea832 100644
--- a/arch/loongarch/kernel/fpu.S
+++ b/arch/loongarch/kernel/fpu.S
@@ -22,7 +22,7 @@
.macro EX insn, reg, src, offs
.ex\@: \insn \reg, \src, \offs
- _asm_extable .ex\@, fault
+ _asm_extable .ex\@, .L_fixup_fpu_fault
.endm
.macro sc_save_fp base
@@ -521,7 +521,6 @@ SYM_FUNC_START(_restore_lasx_context)
jr ra
SYM_FUNC_END(_restore_lasx_context)
-SYM_FUNC_START(fault)
+.L_fixup_fpu_fault:
li.w a0, -EFAULT # failure
jr ra
-SYM_FUNC_END(fault)
diff --git a/arch/loongarch/kernel/lbt.S b/arch/loongarch/kernel/lbt.S
index ecf08bbff650..402eb8ec4024 100644
--- a/arch/loongarch/kernel/lbt.S
+++ b/arch/loongarch/kernel/lbt.S
@@ -16,7 +16,7 @@
.macro EX insn, reg, src, offs
.ex\@: \insn \reg, \src, \offs
- _asm_extable .ex\@, lbt_fault
+ _asm_extable .ex\@, .L_fixup_lbt_fault
.endm
/*
@@ -150,7 +150,6 @@ SYM_FUNC_START(_restore_ftop_context)
jr ra
SYM_FUNC_END(_restore_ftop_context)
-SYM_FUNC_START(lbt_fault)
+.L_fixup_lbt_fault:
li.w a0, -EFAULT # failure
jr ra
-SYM_FUNC_END(lbt_fault)
Thanks,
Tiezhu