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work around gcc bugs with 'asm goto' with outputs

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commit 4356e9f841f7fbb945521cef3577ba394c65f3fc upstream.

We've had issues with gcc and 'asm goto' before, and we created a
'asm_volatile_goto()' macro for that in the past: see commits
3f0116c323 ("compiler/gcc4: Add quirk for 'asm goto' miscompilation
bug") and a9f180345f ("compiler/gcc4: Make quirk for
asm_volatile_goto() unconditional").

Then, much later, we ended up removing the workaround in commit
43c249ea0b ("compiler-gcc.h: remove ancient workaround for gcc PR
58670") because we no longer supported building the kernel with the
affected gcc versions, but we left the macro uses around.

Now, Sean Christopherson reports a new version of a very similar
problem, which is fixed by re-applying that ancient workaround.  But the
problem in question is limited to only the 'asm goto with outputs'
cases, so instead of re-introducing the old workaround as-is, let's
rename and limit the workaround to just that much less common case.

It looks like there are at least two separate issues that all hit in
this area:

 (a) some versions of gcc don't mark the asm goto as 'volatile' when it
     has outputs:

        https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98619
        https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110420

     which is easy to work around by just adding the 'volatile' by hand.

 (b) Internal compiler errors:

        https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110422

     which are worked around by adding the extra empty 'asm' as a
     barrier, as in the original workaround.

but the problem Sean sees may be a third thing since it involves bad
code generation (not an ICE) even with the manually added 'volatile'.

but the same old workaround works for this case, even if this feels a
bit like voodoo programming and may only be hiding the issue.

Reported-and-tested-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/all/20240208220604.140859-1-seanjc@google.com/
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: Jakub Jelinek <jakub@redhat.com>
Cc: Andrew Pinski <quic_apinski@quicinc.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Linus Torvalds
2024-02-09 12:39:31 -08:00
committed by Greg Kroah-Hartman
parent b2c9bf0647
commit aaff74d886
31 changed files with 84 additions and 65 deletions
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19
include/linux/compiler-gcc.h
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@@ -66,6 +66,25 @@
__builtin_unreachable(); \
} while (0)
/*
* GCC 'asm goto' with outputs miscompiles certain code sequences:
*
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110420
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110422
*
* Work it around via the same compiler barrier quirk that we used
* to use for the old 'asm goto' workaround.
*
* Also, always mark such 'asm goto' statements as volatile: all
* asm goto statements are supposed to be volatile as per the
* documentation, but some versions of gcc didn't actually do
* that for asms with outputs:
*
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98619
*/
#define asm_goto_output(x...) \
do { asm volatile goto(x); asm (""); } while (0)
#if defined(CONFIG_ARCH_USE_BUILTIN_BSWAP)
#define __HAVE_BUILTIN_BSWAP32__
#define __HAVE_BUILTIN_BSWAP64__