Add ar --output

[Fangrui Song]

On 2019-11-04, Nick Clifton wrote:
>Hi Fangrui,
>
>> Thanks for pushing this, however, I think the landed commit does not
>> support absolute paths or paths that contain ..
>>
>> Â ar --output=/tmp x a.a file
>> Â ar --output=../dir x a.a file
>
>Right - I had thought that this was what we had decided should happen.
>
>> While I agree that `ar x a.a ../file` and `ar x a.a /tmp/file` may lead
>> to directory traversal vulnerabilities, I think people who specify
>> --output will likely use an absolute path or a path containing ..
>
>OK, so what do you think of the patch below ?
>
>Cheers
>  Nick
>
>
>
> diff --git a/binutils/ar.c b/binutils/ar.c
>index 0af7954a47..2e706fb46a 100644
>--- a/binutils/ar.c
>+++ b/binutils/ar.c
>@@ -1083,10 +1083,9 @@ open_output_file (bfd * abfd)
> 	    output_filename = concat (output_dir, "/", output_filename, NULL);
> 	}
>     }
>-
>   /* PR binutils/17533: Do not allow directory traversal
>      outside of the current directory tree.  */
>-  if (! is_valid_archive_path (output_filename))
>+  else if (! is_valid_archive_path (output_filename))
>     {
>       char * base = (char *) lbasename (output_filename);

Thanks for the patch.

I think the is_valid_archive_path check should be placed before `if (output_dir)`.

printf '!<arch>\n%-48s%-10d`\n../file\n%-48s%-10s`\n' '//' 8 '/0' 0 > test.a
ar --output=/tmp/c x test.a => extracted ../file to /tmp/c, i.e.  /tmp/file

>diff --git a/binutils/doc/binutils.texi b/binutils/doc/binutils.texi
>index 2a86b92033..11de2d3deb 100644
>--- a/binutils/doc/binutils.texi
>+++ b/binutils/doc/binutils.texi
>@@ -390,7 +390,9 @@ are extracted.
> Files cannot be extracted from a thin archive, and there are
> restrictions on extracting from archives created with @option{P}: The
> paths must not be absolute, may not contain @code{..}, and any
>-subdirectories in the paths must exist.
>+subdirectories in the paths must exist.  If it is desired to avoid
>+these restrictions then used the @option{--output} option to specify
>+an output directory.
> @end table
>
> A number of modifiers (@var{mod}) may immediately follow the @var{p}
>@@ -589,11 +591,6 @@ Note - although the presence of this option does imply a @option{x}
> extraction operation that option must still be included on the command
> line.
>
>-Note - using this option does not allow archive members to be
>-extracted to locations outside of the current directory, or one of its
>-sub-directories.  This is a security feature to prevent archives
>-created with the @option{P} option from maliciously overwriting user
>-files.
> @end table
> @c man end
>