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

[Yann Droneaud <ydroneaud@opteya.com>]

Hi,

Le jeudi 03 octobre 2019 à 19:59 +0200, Andreas Schwab a écrit :
> On Okt 03 2019, Yann Droneaud <ydroneaud@opteya.com> wrote:
> 
> > 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.
> 
> That only happens if you have a note segment that does not fit in the
> initial file buffer.  That is rather unusual.
> 

Not sure about it being unusual: on Fedora 30, x86_64, I see this
behavior with /bin/false /bin/true /bin/test /bin/uname 

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=6697832, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x7f8bc04b5000
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


$ file /bin/false /bin/true /bin/test /bin/uname 
/bin/false: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV),
dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for
GNU/Linux 3.2.0,
BuildID[sha1]=790d23abaaa68bb496873f5afc647dd6be30f94c, stripped, too
many notes (256)
/bin/true:  ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV),
dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for
GNU/Linux 3.2.0,
BuildID[sha1]=8e348e43f020cfa72b8abed7e69b54558165fb2e, stripped, too
many notes (256)
/bin/test:  ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV),
dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for
GNU/Linux 3.2.0,
BuildID[sha1]=5dee19457e437541fe30f58d3d4add118567e806, stripped, too
many notes (256)
/bin/uname: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV),
dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for
GNU/Linux 3.2.0,
BuildID[sha1]=50b398bf94ad5a081eb4178d976d4f290977f99a, stripped, too
many notes (256)

Too many notes ?

Regards.

-- 
Yann Droneaud
OPTEYA