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Message-ID: <CAF1bQ=TA9FZXcXk=eRqN_tHkJztd_BLEd14X8JWu7LoGHMAVMg@mail.gmail.com>
Date: Mon, 3 Nov 2025 13:40:02 -0800
From: Rong Xu <xur@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, linux-kernel@...r.kernel.org,
Sriraman Tallam <tmsriram@...gle.com>, Han Shen <shenhan@...gle.com>,
Krzysztof Pszeniczny <kpszeniczny@...gle.com>
Subject: Re: [PATCH 2/2] objtool: dead_end function change for split functions
Peter, thanks for the review! I will fix the style and send patch v2.
-Rong
On Mon, Nov 3, 2025 at 1:19 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Nov 03, 2025 at 06:51:54PM +0000, xur@...gle.com wrote:
> > From: Rong Xu <xur@...gle.com>
> >
> > Function Splitting can potentially move all return instructions
> > into the cold (infrequently executed) section of the function.
> > If this happens, the original function might be incorrectly
> > flagged as a dead-end function.
> >
> > The consequence is an incomplete ORC table, which leads to an unwind
> > error, and subsequently, a livepatch failure.
> >
> > This patch adds the support of the dead_end_function check for
> > split function.
> >
> > Signed-off-by: Rong Xu <xur@...gle.com>
> > Reviewed-by: Sriraman Tallam <tmsriram@...gle.com>
> > Reviewed-by: Han Shen <shenhan@...gle.com>
> > Reviewed-by: Krzysztof Pszeniczny <kpszeniczny@...gle.com>
> > ---
> > tools/objtool/check.c | 88 +++++++++++++++++++++++++++++++++----------
> > 1 file changed, 69 insertions(+), 19 deletions(-)
> >
> > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > index c2ee3c3a84a62..b752cf508d09a 100644
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -237,6 +237,73 @@ static bool is_rust_noreturn(const struct symbol *func)
> > str_ends_with(func->name, "_fail"));
> > }
> >
> > +static bool __dead_end_function(struct objtool_file *, struct symbol *, int);
> > +
> > +/*
> > + * Check if the target of a sibling_call instruction is a dead_end function.
> > + * Note insn must be a sibling call.
> > + */
> > +static inline bool __dead_end_sibling_call(struct objtool_file *file,
> > + struct instruction *insn, int recursion) {
>
> Please: cino=(0:0
> also for functions { on a new line.
>
> > + struct instruction *dest = insn->jump_dest;
> > +
> > + if (!dest)
> > + /* sibling call to another file */
> > + return false;
>
> I know this is just code movement, but this wants {} per coding style.
>
> > +
> > + /* local sibling call */
> > + if (recursion == 5) {
> > + /*
> > + * Infinite recursion: two functions have
> > + * sibling calls to each other. This is a very
> > + * rare case. It means they aren't dead ends.
> > + */
> > + return false;
> > + }
> > +
> > + return __dead_end_function(file, insn_func(dest), recursion+1);
> > +}
> > +
> > +/*
> > + * Handling split functions. Mimic the workflow in __dead_end_function.
> > + */
> > +static bool __dead_end_split_func(struct objtool_file *file,
> > + struct symbol *func, int recursion)
>
> cino=(0:0
>
> > +{
> > + char section_name[256];
> > + struct section *sec;
> > + struct instruction *insn;
> > +
> > + /*
> > + * Use a fixed-size buffer (max 256) to avoid malloc. If the section
> > + * length exceeds this limit, we return a conservative value. This is
> > + * a safe fallback and does not compromise functional correctness.
> > + */
> > + if (snprintf(section_name, sizeof(section_name), ".text.split.%s",
> > + func->name) >= sizeof(section_name)) {
>
> That is a terribly confusing line-break to read. Might've been better to
> split after the greate-or-equal sign.
>
> > + fprintf(stderr, "Error: Function name '%s' too long.\n", func->name);
> > + return false;
> > + }
> > +
> > + sec = find_section_by_name(file->elf, section_name);
> > + if (!sec)
> > + return false;
> > +
> > + sec_for_each_insn(file, sec, insn) {
> > + if (insn->type == INSN_RETURN)
> > + return false;
> > + }
> > +
> > + sec_for_each_insn(file, sec, insn) {
> > + if (is_sibling_call(insn)) {
> > + if (!__dead_end_sibling_call(file, insn, recursion))
> > + return false;
> > + }
> > + }
> > +
> > + return true;
> > +}
> > +
> > /*
> > * This checks to see if the given function is a "noreturn" function.
> > *
> > @@ -298,33 +365,16 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
> > */
> > func_for_each_insn(file, func, insn) {
> > if (is_sibling_call(insn)) {
> > - struct instruction *dest = insn->jump_dest;
> > -
> > - if (!dest)
> > - /* sibling call to another file */
> > - return false;
> > -
> > - /* local sibling call */
> > - if (recursion == 5) {
> > - /*
> > - * Infinite recursion: two functions have
> > - * sibling calls to each other. This is a very
> > - * rare case. It means they aren't dead ends.
> > - */
> > - return false;
> > - }
> > -
> > /*
> > * A function can have multiple sibling calls. All of
> > * them need to be dead ends for the function to be a
> > * dead end too.
> > */
> > - if (!__dead_end_function(file, insn_func(dest), recursion+1))
> > + if (!__dead_end_sibling_call(file, insn, recursion))
> > return false;
> > }
> > }
> > -
> > - return true;
> > + return __dead_end_split_func(file, func, recursion);
> > }
>
> Aside from some coding style nits, this seems like it will do.
>
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