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    SubjectRE: [RFC PATCH v1 3/6] tools/nolibc: i386: Implement syscall with 6 arguments
    Date
    From: Ammar Faizi
    > Sent: 20 March 2022 09:38
    >
    > In i386, the 6th argument of syscall goes in %ebp. However, both Clang
    > and GCC cannot use %ebp in the clobber list and in the "r" constraint
    > without using -fomit-frame-pointer. To make it always available for any
    > kind of compilation, the below workaround is implemented.
    >
    > For clang (the Assembly statement can't clobber %ebp):
    > 1) Save the %ebp value to the redzone area -4(%esp).

    i386 doesn't have a redzone.
    If you get a signal it will trash -4(%sp)

    > 2) Load the 6-th argument from memory to %ebp.
    > 3) Subtract the %esp by 4.
    > 4) Do the syscall (int $0x80).
    > 5) Pop %ebp.
    >
    > For GCC, fortunately it has a #pragma that can force a specific function
    > to be compiled with -fomit-frame-pointer, so it can always use "r"(var)
    > where `var` is a variable bound to %ebp.

    How is that going to work for an inlined functon?

    And using xchg is slow - it is always locked.

    One possibility might be to do:
    push arg6
    push %ebp
    mov %ebp, 4(%sp)
    int 0x80
    pop %ebp
    add %esp,4

    Although I'm not sure you really want to allocate 4k pages
    for every malloc() call.

    Probably better to write a mini 'libc' that uses sbrk()
    and a best fit scan of a linear free list.

    David

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