Le 14/02/2021 à 12:24, Alex Bennée a écrit :
>
> Vincent Fazio <vfa...@xes-inc.com> writes:
>
>> From: Vincent Fazio <vfa...@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.
But what is the purpose of the "if (MAP_FIXED_NOREPLACE != 0 ||"?
Can't we rely only on "mmap_start == (void *) align_start"?
Thanks,
Laurent
>>
>> 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 <vfa...@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;
>> }
>> }
>
>
