Re: [PATCH 32/48] objtool: Add is_cold_func() helper

Josh Poimboeuf
2026-04-23T19:23:35Z

On Thu, Apr 23, 2026 at 05:14:05PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 23, 2026 at 08:12:07AM -0700, Josh Poimboeuf wrote:
> > On Thu, Apr 23, 2026 at 10:38:49AM +0200, Peter Zijlstra wrote:
> > > On Wed, Apr 22, 2026 at 09:04:00PM -0700, Josh Poimboeuf wrote:
> > >
> > > > diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> > > > index 00c2389f345f..8a6e1338af97 100644
> > > > --- a/tools/objtool/elf.c
> > > > +++ b/tools/objtool/elf.c
> > > > @@ -586,8 +586,11 @@ static int elf_add_symbol(struct elf *elf, struct symbol *sym)
> > > > if (strstarts(sym->name, ".klp.sym"))
> > > > sym->klp = 1;
> > > >
> > > > + sym->pfunc = sym->cfunc = sym;
> > > > +
> > > > if (!sym->klp && !is_sec_sym(sym) && strstr(sym->name, ".cold")) {
> > > > - sym->cold = 1;
> > > > + /* Tell read_symbols() this is a cold subfunction */
> > > > + sym->pfunc = NULL;
> > > >
> > > > /*
> > > > * Clang doesn't mark cold subfunctions as STT_FUNC, which
> > > > @@ -596,8 +599,6 @@ static int elf_add_symbol(struct elf *elf, struct symbol *sym)
> > > > sym->type = STT_FUNC;
> > > > }
> > > >
> > > > - sym->pfunc = sym->cfunc = sym;
> > > > -
> > > > return 0;
> > > > }
> > >
> > > So now the cold subfunction has a NULL parent-function and a
> > > child-function that points to the parent?
> > >
> > > I'm confused.
> >
> > It's a bit clunky. As the comment implies, 'sym->pfunc = NULL' is a
> > signal to it caller read_symbols() that this is a .cold function. Then,
> > after all the symbols have been added, read_symbols() goes and finds the
> > parent.
> >
> > I think I did it this way because klp-diff.c calls elf_add_symbol() (via
> > elf_create_symbol()) and later needs to call is_cold_func() on it. In
> > that case, even though the parent isn't set, it still works because
> > is_cold_func() returns true for sym->pfunc != sym;
>
> I'm thinking this needs more comments if it stays like this. Is most
> confusing.

So we can just keep the 'cold' bit and keep the confusingness at its
current level :-)

From: Josh Poimboeuf <jpoimboe@kernel.org>
Subject: [PATCH] objtool: Add is_cold_func() helper

Add an is_cold_func() helper. No functional changes intended.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
tools/objtool/check.c | 6 +++---
tools/objtool/include/objtool/elf.h | 5 +++++
tools/objtool/klp-diff.c | 3 ++-
3 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 4c18d6e7f6c3..4ed27c53c718 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2614,7 +2614,7 @@ static void mark_holes(struct objtool_file *file)
if (insn->jump_dest) {
struct symbol *dest_func = insn_func(insn->jump_dest);

- if (dest_func && dest_func->cold)
+ if (dest_func && is_cold_func(dest_func))
dest_func->ignore = true;
}
}
@@ -4426,8 +4426,8 @@ static int create_prefix_symbol(struct objtool_file *file, struct symbol *func)
char name[SYM_NAME_LEN];
struct cfi_state *cfi;

- if (!is_func_sym(func) || is_prefix_func(func) ||
- func->cold || func->static_call_tramp)
+ if (!is_func_sym(func) || is_prefix_func(func) || is_cold_func(func) ||
+ func->static_call_tramp)
return 0;

if ((strlen(func->name) + sizeof("__pfx_") > SYM_NAME_LEN)) {
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index 3abe4cbc584c..ad0cc57a9d5f 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -289,6 +289,11 @@ static inline bool is_prefix_func(struct symbol *sym)
return sym->prefix;
}

+static inline bool is_cold_func(struct symbol *sym)
+{
+ return sym->cold;
+}
+
static inline bool is_reloc_sec(struct section *sec)
{
return sec->sh.sh_type == SHT_RELA || sec->sh.sh_type == SHT_REL;
diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index 1951a8b2df44..266f0d2ba4fe 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -1718,7 +1718,8 @@ static int create_klp_sections(struct elfs *e)
unsigned long sympos;
void *func_data;

- if (!is_func_sym(sym) || sym->cold || !sym->clone || !sym->clone->changed)
+ if (!is_func_sym(sym) || is_cold_func(sym) ||
+ !sym->clone || !sym->clone->changed)
continue;

/* allocate klp_func_ext */
--
2.53.0