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

[Laurent Vivier]

Le 14/02/2021 à 12:24, Alex Bennée a écrit :
> 
> 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.


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 <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;
>>                  }
>>              }
> 
>