[PATCH 2/2] elf: Use nocancel pread64() instead of lseek()+read()

[Yann Droneaud]

Hi,

Le lundi 05 août 2019 à 21:56 +0000, Leandro Pereira a écrit :
> Transforms this, when linking in a shared object:
> 
>   openat(AT_FDCWD, "/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
>   read(3, "\177ELF\2\1\1\3"..., 832) = 832
>   lseek(3, 792, SEEK_SET)           = 792
>   read(3, "\4\0\0\0\24\0\0\0"..., 68) = 68
>   fstat(3, {st_mode=S_IFREG|0755, st_size=6699224, ...}) = 0
>   lseek(3, 792, SEEK_SET)           = 792
>   read(3, "\4\0\0\0\24\0\0\0"..., 68) = 68
>   lseek(3, 864, SEEK_SET)           = 864
>   read(3, "\4\0\0\0\20\0\0\0"..., 32) = 32
> 
> Into this:
> 
>   openat(AT_FDCWD, "/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
>   read(3, "\177ELF\2\1\1\3"..., 832) = 832
>   pread(3, "\4\0\0\0\24\0\0\0"..., 68, 792) = 68
>   fstat(3, {st_mode=S_IFREG|0755, st_size=6699224, ...}) = 0
>   pread(3, "\4\0\0\0\24\0\0\0"..., 68, 792) = 68
>   pread(3, "\4\0\0\0\20\0\0\0"..., 32, 864) = 32
> 

I confirm the behavior. And the usefulness of the patch.

Anyway I'm quite surprised ld.so is reading twice 68 bytes at the same
792 offset. Moreover, the first read already brought 28 bytes out of
68.

Could these read() be replaced by a call to mmap(), then using mremap()
to brought the shared object in memory after initial inspection ?

Regards.

-- 
Yann Droneaud
OPTEYA