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SubjectRe: [PATCHv4 RESEND 0/3] syscalls,x86: Add execveat() system call
Andy Lutomirski <> writes:

> [Added Eric Biederman, since I think your tree might be a reasonable
> route forward for these patches.]
> On Thu, Jun 5, 2014 at 6:40 AM, David Drysdale <> wrote:
>> Resending, adding cc:linux-api.
>> Also, it may help to add a little more background -- this patch is
>> needed as a (small) part of implementing Capsicum in the Linux kernel.
>> Capsicum is a security framework that has been present in FreeBSD since
>> version 9.0 (Jan 2012), and is based on concepts from object-capability
>> security [1].
>> One of the features of Capsicum is capability mode, which locks down
>> access to global namespaces such as the filesystem hierarchy. In
>> capability mode, /proc is thus inaccessible and so fexecve(3) doesn't
>> work -- hence the need for a kernel-space
> I just found myself wanting this syscall for another reason: injecting
> programs into sandboxes or otherwise heavily locked-down namespaces.
> For example, I want to be able to reliably do something like nsenter
> --namespace-flags-here toybox sh. Toybox's shell is unusual in that
> it is more or less fully functional, so this should Just Work (tm),
> except that the toybox binary might not exist in the namespace being
> entered. If execveat were available, I could rig nsenter or a similar
> tool to open it with O_CLOEXEC, enter the namespace, and then call
> execveat.
> Is there any reason that these patches can't be merged more or less as
> is for 3.19?

Yes. There is a silliness in how it implements fexecve. The fexecve
case should be use the empty string "" not a NULL pointer to indication
that. That change will then harmonize execveat with the other
system calls and simplify the code and remove a special case. I believe
using the empty string "" requires implementing the AT_EMPTY_PATH flag.

For sandboxes execveat seems to make a great deal of sense. I can
get the same functionality by passing in a directory file descriptor
calling fchdir and execve so this should not introduce any new security
holes. And using the final file descriptor removes a race.

AT_SYMLINK_NOFOLLOW seems to have some limited utility as well, although
for exec I don't know what problems it can solve.

Until I am done moving I won't have time to pick this up, and the code
clearly needs another revision but I will be happy to work to see that
we get a sane execveat implemented.


p.s. I don't believe there are any namespaces issues where doing
something with execveat flags make sense.

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