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

[Carlos O'Donell <carlos at redhat dot com>]

On 8/5/19 5:56 PM, Leandro Pereira wrote:
> 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
> 
> 2019-08-05  Leandro Pereira  <leandro.pereira@microsoft.com>
> 
> 	* elf/dl-load.c: Use __pread64_nocancel() instead of __lseek()+
> 	__read_nocancel().
> 	* sysdeps/x86/dl-prop.h: Likewise.

OK for master. I'll push after testing with bmg. No regressions on x86_64.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> ---
>  elf/dl-load.c         | 9 +++------
>  sysdeps/x86/dl-prop.h | 3 +--
>  2 files changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index 5abeb867f1..462c425a13 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -1005,8 +1005,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
>    else
>      {

OK. I reviewed that all uses of fd did not depend on the SEEK_SET being set to
a given position.

>        phdr = alloca (maplength);
> -      __lseek (fd, header->e_phoff, SEEK_SET);
> -      if ((size_t) __read_nocancel (fd, (void *) phdr, maplength) != maplength)
> +      if ((size_t) __pread64_nocancel (fd, (void *) phdr, maplength, header->e_phoff) != maplength)

This line is too long, but I'll wrap it before commit, likewise for 2 other lines.

>  	{
>  	  errstring = N_("cannot read file data");
>  	  goto call_lose_errno;
> @@ -1659,8 +1658,7 @@ open_verify (const char *name, int fd,
>        else
>  	{
>  	  phdr = alloca (maplength);
> -	  __lseek (fd, ehdr->e_phoff, SEEK_SET);
> -	  if ((size_t) __read_nocancel (fd, (void *) phdr, maplength)
> +	  if ((size_t) __pread64_nocancel (fd, (void *) phdr, maplength, ehdr->e_phoff)

OK. Wrapped.

>  	      != maplength)
>  	    {
>  	    read_error:
> @@ -1710,8 +1708,7 @@ open_verify (const char *name, int fd,
>  
>  		    abi_note = abi_note_malloced;
>  		  }
> -		__lseek (fd, ph->p_offset, SEEK_SET);
> -		if (__read_nocancel (fd, (void *) abi_note, size) != size)
> +		if (__pread64_nocancel (fd, (void *) abi_note, size, ph->p_offset) != size)

OK. Wrapped.

>  		  {
>  		    free (abi_note_malloced);
>  		    goto read_error;
> diff --git a/sysdeps/x86/dl-prop.h b/sysdeps/x86/dl-prop.h
> index 1b335ccbb3..080d66a971 100644
> --- a/sysdeps/x86/dl-prop.h
> +++ b/sysdeps/x86/dl-prop.h
> @@ -167,8 +167,7 @@ _dl_process_pt_note (struct link_map *l, const ElfW(Phdr) *ph,
>  	  note_malloced = malloc (size);
>  	  note = note_malloced;
>  	}
> -      __lseek (fd, ph->p_offset, SEEK_SET);
> -      if (__read_nocancel (fd, (void *) note, size) != size)
> +      if (__pread64_nocancel (fd, (void *) note, size, ph->p_offset) != size)

OK.

>  	{
>  	  if (note_malloced)
>  	    free (note_malloced);
> 


-- 
Cheers,
Carlos.