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    Subject[PATCH 5.15 030/846] tools/nolibc: i386: fix initial stack alignment
    Date
    From: Willy Tarreau <w@1wt.eu>

    commit ebbe0d8a449d183fa43b42d84fcb248e25303985 upstream.

    After re-checking in the spec and comparing stack offsets with glibc,
    The last pushed argument must be 16-byte aligned (i.e. aligned before the
    call) so that in the callee esp+4 is multiple of 16, so the principle is
    the 32-bit equivalent to what Ammar fixed for x86_64. It's possible that
    32-bit code using SSE2 or MMX could have been affected. In addition the
    frame pointer ought to be zero at the deepest level.

    Link: https://gitlab.com/x86-psABIs/i386-ABI/-/wikis/Intel386-psABI
    Cc: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
    Cc: stable@vger.kernel.org
    Signed-off-by: Willy Tarreau <w@1wt.eu>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    tools/include/nolibc/nolibc.h | 10 +++++++++-
    1 file changed, 9 insertions(+), 1 deletion(-)

    --- a/tools/include/nolibc/nolibc.h
    +++ b/tools/include/nolibc/nolibc.h
    @@ -583,13 +583,21 @@ struct sys_stat_struct {
    })

    /* startup code */
    +/*
    + * i386 System V ABI mandates:
    + * 1) last pushed argument must be 16-byte aligned.
    + * 2) The deepest stack frame should be set to zero
    + *
    + */
    asm(".section .text\n"
    ".global _start\n"
    "_start:\n"
    "pop %eax\n" // argc (first arg, %eax)
    "mov %esp, %ebx\n" // argv[] (second arg, %ebx)
    "lea 4(%ebx,%eax,4),%ecx\n" // then a NULL then envp (third arg, %ecx)
    - "and $-16, %esp\n" // x86 ABI : esp must be 16-byte aligned when
    + "xor %ebp, %ebp\n" // zero the stack frame
    + "and $-16, %esp\n" // x86 ABI : esp must be 16-byte aligned before
    + "sub $4, %esp\n" // the call instruction (args are aligned)
    "push %ecx\n" // push all registers on the stack so that we
    "push %ebx\n" // support both regparm and plain stack modes
    "push %eax\n"

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