Re: [PULL 08/16] linux-user: don't use MAP_FIXED in pgd_find_hole_fallback

[Peter Maydell]

On Mon, 27 Jul 2020 at 13:24, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> Plain MAP_FIXED has the undesirable behaviour of splatting exiting
> maps so we don't actually achieve what we want when looking for gaps.
> We should be using MAP_FIXED_NOREPLACE. As this isn't always available
> we need to potentially check the returned address to see if the kernel
> gave us what we asked for.
>
> Fixes: ad592e37dfc ("linux-user: provide fallback pgd_find_hole for bare 
> chroots")
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> Message-Id: <20200724064509.331-9-alex.bennee@linaro.org>

Hi; Coverity thinks this conditional expression is suspicious
(CID 1431059):

>              if (mmap_start != MAP_FAILED) {
>                  munmap((void *) align_start, guest_size);
> -                return (uintptr_t) mmap_start + offset;
> +                if (MAP_FIXED_NOREPLACE || mmap_start == (void *) 
> align_start) {

because it's performing a logical OR operation where the left
operand is an integer constant that's neither 0 nor 1
(it's 1048576). What was this intended to be?

> +                    return (uintptr_t) mmap_start + offset;
> +                }
>              }

thanks
-- PMM