sbrk vs mmap

[Suleiman Souhlal]

darran kartaschew wrote:
>    Hi Guys,
>    I'm having some issues with rewriting a simple malloc() function to be
>    with FreeBSD (AMD64). This is part of porting an application from
>    Linux
>    to FreeBSD.
>    After pulling my hair out for a while, I've found that the sbrk()
>    system call just returns "45 - Operation Not Supported" error,
>    irrespective of the parameters passed to it. (I've found the source
>    for sbrk() and see that it's not implemented).
>    So I decided to try using mmap() instead. All memory allocations don't
>    have to be continuous, so mmap() will suffice. The problem is I'm
>    getting an invalid file handle error? According to the man page, if
>    you
>    use MAP_ANON you're just allocating a block of memory without linking
>    to a file, and a handle of -1 should be supplied... Any way code is as
>    follows:
>    memInit:
>    mov r4, 0 ; don't care where the memory is allocated
>    mov r5, 1048576 ; alloc 1MB
>    mov r3, 3 ; RW access to memory
>    mov r2, 4096 ; MAP_ANON - not a file
>    mov r8d, -1 ; -1 for file handle if using MAP_ANON
>    mov r9, 0 ; ignored for MAP_ANON
>    mov r0, 197 ; mmap();
>    syscall
>    mov qword [_mmap], r0 ; save address so we can release it on exit;
>    ret
>    It fails with an EBADF (9) ; Bad File Descriptor error...
>    Note: r0 = rax, r1 = rbx, r2 = rcx, r3 = rdx, r4 = rdi, r5 = rsi, r6 =
>    rbp, r7 = rsp. Various parameters for mmap() are found in mman.h>.
>    So does anyone have an example of a working call to mmap() or tell me
>    what's wrong with the above code?
>    I've done up a test C program that simple calls mmap(), after
>    tracing through the compiled C program using gdb I can't see that
>    I'm doing anything different to what gcc/glibc are doing? (except
>    the macro expansion that's in libc which adds an additional
>    0 to the top of the stack).
>    PS. FASM 1.66 running on FreeBSD 6.1 (AMD64).
>    PPS. This is NOT a homework assignment! (tm) :P

I think you are missing a parameter.
mmap (as well as pwrite, lseek, truncate and ftruncate), has a "hidden"
parameter just before the offset that is ignored, due to a bug in ancient
GCC versions. So, basically, you should also push a 0 on the stack.
Take a look at src/sys/libc/sys/mmap.c .
I have a patch to remove this useless argument, but haven't committed it
yet.

-- Suleiman