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Message-ID: <bzy7cad37tafrbcmsstn355fpljxxmi25ifc4piihp6ln3ztxh@zp3c7ydsjmuq>
Date: Mon, 7 Apr 2025 18:23:25 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Philip Li <philip.li@...el.com>, kernel test robot <lkp@...el.com>,
Guenter Roeck <linux@...ck-us.net>, oe-kbuild-all@...ts.linux.dev,
Andrew Morton <akpm@...ux-foundation.org>, Linux Memory Management List <linux-mm@...ck.org>,
Alessandro Carminati <acarmina@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org, loongarch@...ts.linux.dev
Subject: Re: [linux-next:master 12681/13861] drivers/i2c/i2c-core-base.o:
warning: objtool: __i2c_transfer+0x120: stack state mismatch: reg1[24]=-1+0
reg2[24]=-2-24
On Mon, Apr 07, 2025 at 06:52:10PM +0800, Tiezhu Yang wrote:
> There is a potential execution path with only using s0 and ra
> (without using s1, s2, s3, etc): 2d58-->2d70-->2f88-->2e78-->2e84
[...]
> From this point of view, it seems that there is no problem for the
> generated instructions of the current code, it is not a runtime bug,
> just a GCC optimization.
I don't see how this is responsive to my email.
I described a code path which revealed a GCC bug, specifically with asm
goto (unless I got something wrong). Then you responded with a
*completely different* code path.
How does that prove my original code path isn't possible?
To summarize, the path I found was
2d58 ... 2d9c -> 2da8 .. 2dc4 -> 2ebc .. 2ec0 (runtime patched static branch) -> 2e78 .. 2e84 (ret)
> (2) Analysis
>
> In fact, the generated objtool warning is because the break instruction
> (2ee8) which is before the restoring s1 instruction (2eec) is annotated
> as dead end.
Actually, it's the opposite. Objtool would normally consider BREAK to
be a dead end. But it's annotated as "reachable", aka "non dead end".
> This issue is introduced by the following changes:
>
> #define __WARN_FLAGS(flags) \
> do { \
> instrumentation_begin(); \
> - __BUG_FLAGS(BUGFLAG_WARNING|(flags), ANNOTATE_REACHABLE(10001b));\
> + if (!KUNIT_IS_SUPPRESSED_WARNING(__func__)) \
> + __BUG_FLAGS(BUGFLAG_WARNING|(flags), ANNOTATE_REACHABLE(10001b));\
> instrumentation_end(); \
> } while (0)
>
> of commit e61a8b4b0d83 ("loongarch: add support for suppressing warning
> backtraces") in the linux-next.git.
Putting that annotation behind a conditional should not break anything.
> (4) Solution 1
> One way is to annotate __BUG_ENTRY() as reachable whether
> KUNIT_IS_SUPPRESSED_WARNING() is true or false, like this:
>
> ---8<---
> diff --git a/arch/loongarch/include/asm/bug.h
> b/arch/loongarch/include/asm/bug.h
> index b79ff6696ce6..e41ebeaba204 100644
> --- a/arch/loongarch/include/asm/bug.h
> +++ b/arch/loongarch/include/asm/bug.h
> @@ -60,8 +60,9 @@
> #define __WARN_FLAGS(flags) \
> do { \
> instrumentation_begin(); \
> - if (!KUNIT_IS_SUPPRESSED_WARNING(__func__)) \
> - __BUG_FLAGS(BUGFLAG_WARNING|(flags),
> ANNOTATE_REACHABLE(10001b));\
> + if (!KUNIT_IS_SUPPRESSED_WARNING(__func__)) \
> + __BUG_FLAGS(BUGFLAG_WARNING|(flags), ""); \
> + __BUG_FLAGS(0, ANNOTATE_REACHABLE(10001b)); \
> instrumentation_end(); \
> } while (0)
Huh? That's basically:
if (!suppress_warning)
WARN();
BUG();
So it upgrades a conditional WARN to an unconditional BUG???
Not to mention the reachable annotations are backwards: the WARN() is
annotated as dead end while the BUG() is annotated reachable.
Even if that silences objtool somehow, it will most definitely have the
wrong runtime behavior.
> (5) Solution 2
> The other way is to use "-fno-shrink-wrap" to aovid such issue under
> CONFIG_OBJTOOL at compile-time, like this:
As far as I can tell, that would be a workaround to get objtool to stop
warning about a legitimate compiler bug.
--
Josh
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