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Subject: Re: [PATCH 2/2] elf: Use nocancel pread64() instead of lseek()+read()
To: Yann Droneaud <ydroneaud@opteya.com>,
 "libc-alpha@sourceware.org" <libc-alpha@sourceware.org>
Cc: Leandro Pereira <Leandro.Pereira@microsoft.com>
References: <MWHPR2101MB073287753F9E8D211AF49DD3E8DA0@MWHPR2101MB0732.namprd21.prod.outlook.com>
 <ef6eb5407fef7246cf8bc2627c0541ca8d86f94e.camel@opteya.com>
From: Carlos O'Donell <carlos@redhat.com>
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Date: Thu, 03 Oct 2019 17:10:00 -0000
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On 10/3/19 11:31 AM, Yann Droneaud wrote:
> Hi,
>=20
> Le lundi 05 ao=C3=BBt 2019 =C3=A0 21:56 +0000, Leandro Pereira a =C3=A9cr=
it :
>> Transforms this, when linking in a shared object:
>>
>>   openat(AT_FDCWD, "/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) =3D 3
>>   read(3, "\177ELF\2\1\1\3"..., 832) =3D 832
>>   lseek(3, 792, SEEK_SET)           =3D 792
>>   read(3, "\4\0\0\0\24\0\0\0"..., 68) =3D 68
>>   fstat(3, {st_mode=3DS_IFREG|0755, st_size=3D6699224, ...}) =3D 0
>>   lseek(3, 792, SEEK_SET)           =3D 792
>>   read(3, "\4\0\0\0\24\0\0\0"..., 68) =3D 68
>>   lseek(3, 864, SEEK_SET)           =3D 864
>>   read(3, "\4\0\0\0\20\0\0\0"..., 32) =3D 32
>>
>> Into this:
>>
>>   openat(AT_FDCWD, "/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) =3D 3
>>   read(3, "\177ELF\2\1\1\3"..., 832) =3D 832
>>   pread(3, "\4\0\0\0\24\0\0\0"..., 68, 792) =3D 68
>>   fstat(3, {st_mode=3DS_IFREG|0755, st_size=3D6699224, ...}) =3D 0
>>   pread(3, "\4\0\0\0\24\0\0\0"..., 68, 792) =3D 68
>>   pread(3, "\4\0\0\0\20\0\0\0"..., 32, 864) =3D 32
>>
>=20
> I confirm the behavior. And the usefulness of the patch.
>=20
> 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.
>=20
> Could these read() be replaced by a call to mmap(), then using mremap()
> to brought the shared object in memory after initial inspection ?

It might be possible. It's an optimization that would probably require
some refactoring and review of the current logic in the DSO loading.

Happy to review patches :-)

--=20
Cheers,
Carlos.