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    SubjectRe: [PATCH] x86/boot/compressed/64: fix warning for 32-bit trampoline copy
    On Thu, Feb 22, 2018 at 12:02 PM, Kirill A. Shutemov
    <kirill@shutemov.name> wrote:
    > On Thu, Feb 22, 2018 at 11:40:41AM +0100, Arnd Bergmann wrote:
    >> gcc-8 warns that we copy TRAMPOLINE_32BIT_CODE_SIZE bytes from the pointer
    >> to the function into actual trampoline, when that pointer is only 8 bytes:
    >>
    >> In file included from arch/x86/boot/compressed/pgtable_64.c:3:
    >> arch/x86/boot/compressed/pgtable_64.c: In function 'paging_prepare':
    >> arch/x86/boot/compressed/../string.h:18:23: error: '__builtin_memcpy' reading 96 bytes from a region of size 8 [-Werror=stringop-overflow=]
    >> #define memcpy(d,s,l) __builtin_memcpy(d,s,l)
    >> ^~~~~~~~~~~~~~~~~~~~~~~
    >> arch/x86/boot/compressed/pgtable_64.c:62:2: note: in expansion of macro 'memcpy'
    >> memcpy(trampoline + TRAMPOLINE_32BIT_CODE_OFFSET / sizeof(unsigned long),
    >> ^~~~~~
    >>
    >> I assume what was intended here is to copy the trampoline itself rather than
    >> the pointer to the trampoline.
    >>
    >> Cc: Martin Sebor <msebor@gmail.com>
    >> Fixes: b91993a87aff ("x86/boot/compressed/64: Prepare trampoline memory")
    >> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
    >> ---
    >> This is a fairly new warning in gcc, it's possible that the code is
    >> correct and the warning message got it wrong, please double-check this.
    >>
    >> Was the 32-bit trampoline code in the 64-bit decompressor tested successfully?
    >
    > Yes, it was. Actually it tested on every boot of any 64-bit machine.
    >
    > The fix is wrong and it breaks boot. The bug is in trampoline_32bit_src
    > declaration. Could you check if this helps with the warning:

    Got it, yes that definitely explains the problem. Your patch fixes it
    and I'll follow up with a new submission using this change, unless you
    want to send it as a patch yourself.

    Arnd

    > diff --git a/arch/x86/boot/compressed/pgtable.h b/arch/x86/boot/compressed/pgtable.h
    > index 6e0db2260147..5e0b1c4abef4 100644
    > --- a/arch/x86/boot/compressed/pgtable.h
    > +++ b/arch/x86/boot/compressed/pgtable.h
    > @@ -12,7 +12,7 @@
    >
    > #ifndef __ASSEMBLER__
    >
    > -extern void (*trampoline_32bit_src)(void *return_ptr);
    > +extern void trampoline_32bit_src(void *return_ptr);
    >
    > #endif /* __ASSEMBLER__ */
    > #endif /* BOOT_COMPRESSED_PAGETABLE_H */
    > --
    > Kirill A. Shutemov

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