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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Rob Landley <rob@landley.net>
Cc:     linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Ungerer <gerg@linux-m68k.org>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        linux-fsdevel@vger.kernel.org, Al Viro <viro@ZenIV.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Casey Schaufler <casey@schaufler-ca.com>,
        linux-security-module@vger.kernel.org,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Andy Lutomirski <luto@amacapital.net>
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Date:   Wed, 20 May 2020 11:05:29 -0500
In-Reply-To: <adaced72-d757-e3e4-cfeb-5512533d0aa5@landley.net> (Rob Landley's
        message of "Tue, 19 May 2020 16:59:06 -0500")
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Subject: Re: [PATCH v2 7/8] exec: Generic execfd support
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Rob Landley <rob@landley.net> writes:

> On 5/18/20 7:33 PM, Eric W. Biederman wrote:
>> 
>> Most of the support for passing the file descriptor of an executable
>> to an interpreter already lives in the generic code and in binfmt_elf.
>> Rework the fields in binfmt_elf that deal with executable file
>> descriptor passing to make executable file descriptor passing a first
>> class concept.
>
> I was reading this to try to figure out how to do execve(NULL, argv[], envp) to
> re-exec self after a vfork() in a chroot with no /proc, and hit the most trivial
> quibble ever:

We have /proc/self/exe today.  If I understand you correctly you would
like to do the equivalent of 'execve("/proc/self/exe", argv[], envp[])'
without having proc mounted.

The file descriptor is stored in mm->exe_file.

Probably the most straight forward implementation is to allow
execveat(AT_EXE_FILE, ...).

You can look at binfmt_misc for how to reopen an open file descriptor.

>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1323,7 +1323,10 @@ int begin_new_exec(struct linux_binprm * bprm)
>>  	 */
>>  	set_mm_exe_file(bprm->mm, bprm->file);
>>  
>> +	/* If the binary is not readable than enforce mm->dumpable=0 */
>
> then

It took me a minute yes good catch.

Eric