Re: [PATCH] linux-user/elfload: do not assume MAP_FIXED_NOREPLACE kernel support

[Alex Bennée]

Vincent Fazio <vfazio@xes-inc.com> writes:

> From: Vincent Fazio <vfazio@gmail.com>
>
> Previously, pgd_find_hole_fallback assumed that if the build host's libc
> had MAP_FIXED_NOREPLACE defined that the address returned by mmap would
> match the requested address. This is not a safe assumption for Linux
> kernels prior to 4.17

It doesn't as we have in osdep.h:

  #ifndef MAP_FIXED_NOREPLACE
  #define MAP_FIXED_NOREPLACE 0
  #endif

which is to say to assume if MAP_FIXED_NOREPLACE is defined the kernel
should have given us what we want otherwise we do the check.

>
> Now, we always compare mmap's resultant address with the requested
> address and no longer short-circuit based on MAP_FIXED_NOREPLACE.
>
> Fixes: 2667e069e7b5 ("linux-user: don't use MAP_FIXED in 
> pgd_find_hole_fallback")
> Signed-off-by: Vincent Fazio <vfazio@gmail.com>
> ---
>  linux-user/elfload.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> index 5f5f23d2e5..8d425f9ed0 100644
> --- a/linux-user/elfload.c
> +++ b/linux-user/elfload.c
> @@ -2217,8 +2217,7 @@ static uintptr_t pgd_find_hole_fallback(uintptr_t 
> guest_size, uintptr_t brk,
>                                       PROT_NONE, flags, -1, 0);
>              if (mmap_start != MAP_FAILED) {
>                  munmap(mmap_start, guest_size);
> -                if (MAP_FIXED_NOREPLACE != 0 ||
> -                    mmap_start == (void *) align_start) {
> +                if (mmap_start == (void *) align_start) {
>                      return (uintptr_t) mmap_start + offset;
>                  }
>              }


-- 
Alex Bennée