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Date: Tue, 29 Aug 2023 20:47:32 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Xi Ruoyao <xry111@...111.site>, loongarch@...ts.linux.dev,
linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn
Subject: Re: [PATCH] LoongArch: Define the symbol fault as a local label in fpu.S
On Tue, Aug 29, 2023 at 8:40 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
>
>
> 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)
You can only fix the fpu.S, lbt.S part can be squashed to its own patch.
Huacai
> 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
>
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