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Message-ID: <20241008072502.GC14587@noisy.programming.kicks-ass.net>
Date: Tue, 8 Oct 2024 09:25:02 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: bp@...en8.de, david.kaplan@....com, linux-kernel@...r.kernel.org,
x86@...nel.org
Subject: Re: [RFC][PATCH 2/2] x86: Clean up default rethunk warning
On Mon, Oct 07, 2024 at 10:33:45AM -0700, Josh Poimboeuf wrote:
> On Mon, Oct 07, 2024 at 10:32:12AM +0200, Peter Zijlstra wrote:
> > Replace the funny __warn_thunk thing with a more regular
> > WARN_ON_ONCE(), and simplify the ifdeffery.
> >
> > Notably this avoids RET from having recursive RETs (once from the
> > thunk and once from the C function) -- recursive RET makes my head
> > hurt for no good reason.
>
> This could use an explanation for why the ifdefs can be removed and why
> the alternative can be removed.
The alternative is in the WARN_ONCE now.
> > +#define WARN_ONCE \
> > + 1: ALTERNATIVE "", "ud2", X86_FEATURE_ALWAYS ; \
> > + ASM_BUGTABLE_FLAGS(1b, 0, 0, BUGFLAG_WARNING | BUGFLAG_ONCE) ; \
> > + REACHABLE
>
> Can we not use __FILE__ and __LINE__ here?
Because for asm, __FILE__ is spelled like:
#ifdef CONFIG_DEBUG_BUGVERBOSE
.pushsection .rodata.str1.1, "aMS",@progbits,1
.LC0:
.string __FILE__
.popsection
#endif
1: ALTERNATIVE "", "ud2", X86_FEATURE_ALWAYS
ASM_BUGTABLE_FLAGS(1b, LC0b, __LINE__, BUGFLAG_WARNING | BUGFLAG_ONCE)
REACHABLE
And I didn't feel the whole thing was worth the trouble, if NULL bug
will only print the symbol name and that should be clear enough.
> Also why not put this in asm/bug.h?
Because the ALTERNATIVE..
> > SYM_CODE_START(__x86_return_thunk)
> > UNWIND_HINT_FUNC
> > ANNOTATE_NOENDBR
> > -#if defined(CONFIG_MITIGATION_UNRET_ENTRY) || \
> > - defined(CONFIG_MITIGATION_SRSO) || \
> > - defined(CONFIG_MITIGATION_CALL_DEPTH_TRACKING)
> > - ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \
> > - "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS
> > -#else
> > +
> > +#ifdef CONFIG_X86_64
> > + WARN_ONCE
> > +#endif
>
> Isn't this return thunk used before apply_returns()? How does that not
> trigger the warning?
You missed the ALTERNATIVE I squirreled away in the WARN thing :-)
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