linux-kernel - Re: [PATCH v2 2/2] objtool: dead_end function change for split functions

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Message-ID: <2ldfsiw4ebgohtex2o3arpjxtcndxgv2p6omzzqiukk63sn2dr@ew23765buabl>
Date: Mon, 17 Nov 2025 13:50:03 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: xur@...gle.com
Cc: Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, 
	Sriraman Tallam <tmsriram@...gle.com>, Han Shen <shenhan@...gle.com>, 
	Krzysztof Pszeniczny <kpszeniczny@...gle.com>
Subject: Re: [PATCH v2 2/2] objtool: dead_end function change for split
 functions

On Mon, Nov 17, 2025 at 01:43:26PM -0800, Josh Poimboeuf wrote:
> On Mon, Nov 03, 2025 at 09:52:44PM +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.
> 
> I assume there were also some ignored objtool warnings?  Ignoring those
> can have catastrophic consequences (beyond just failing livepatches) so
> I highly recommend building with CONFIG_OBJTOOL_WERROR.
> 
> > 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>
> 
> I was scratching my head about this one for a while.  .text.split.<func>
> is for <func>.cold code, which should automatically get iterated by the
> outer func_for_each_insn() construct in __dead_end_function().  So this
> patch shouldn't be necessary.
> 
> After some testing with Clang -fsplit-machine-functions, I'm realizing
> the problem is that unlike GCC, Clang .cold code (in .text.split.*)
> isn't STT_FUNC.  Which breaks several objtool assumptions.  So something
> like the below is needed.

That last diff had a blatant bug, here is the proper fix:

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 3f20b257ab25..5a9efbda880b 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -502,8 +502,16 @@ static int elf_add_symbol(struct elf *elf, struct symbol *sym)
 	if (strstarts(sym->name, ".klp.sym"))
 		sym->klp = 1;
 
-	if (!sym->klp && is_func_sym(sym) && strstr(sym->name, ".cold"))
+	if (!sym->klp && strstr(sym->name, ".cold")) {
 		sym->cold = 1;
+
+		/*
+		 * Clang doesn't mark cold subfunctions as STT_FUNC, which
+		 * breaks several objtool assumptions.  Fake it.
+		 */
+		sym->type = STT_FUNC;
+	}
+
 	sym->pfunc = sym->cfunc = sym;
 
 	sym->demangled_name = demangle_name(sym);