[PATCHv4 RESEND 0/3] syscalls,x86: Add execveat() system call

From: David Drysdale
Date: Thu Jun 05 2014 - 09:41:01 EST

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 alternative.

[1] http://www.cl.cam.ac.uk/research/security/capsicum/papers/2010usenix-security-capsicum-website.pdf


This patch set adds execveat(2) for x86, and is derived from Meredydd
Luff's patch from Sept 2012 (https://lkml.org/lkml/2012/9/11/528).

The primary aim of adding an execveat syscall is to allow an
implementation of fexecve(3) that does not rely on the /proc
filesystem. The current glibc version of fexecve(3) is implemented
via /proc, which causes problems in sandboxed or otherwise restricted

Given the desire for a /proc-free fexecve() implementation, HPA
suggested (https://lkml.org/lkml/2006/7/11/556) that an execveat(2)
syscall would be an appropriate generalization.

Also, having a new syscall means that it can take a flags argument
without back-compatibility concerns. The current implementation just
defines the AT_SYMLINK_NOFOLLOW flag, but other flags could be added
in future -- for example, flags for new namespaces (as suggested at

Related history:
- https://lkml.org/lkml/2006/12/27/123 is an example of someone
realizing that fexecve() is likely to fail in a chroot environment.
- http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=514043 covered
documenting the /proc requirement of fexecve(3) in its manpage, to
"prevent other people from wasting their time".
- https://bugzilla.kernel.org/show_bug.cgi?id=74481 documented that
it's not possible to fexecve() a file descriptor for a script with
close-on-exec set (which is possible with the implementation here).
- https://bugzilla.redhat.com/show_bug.cgi?id=241609 described a
problem where a process that did setuid() could not fexecve()
because it no longer had access to /proc/self/fd; this has since
been fixed.

Changes since Meredydd's v3 patch:
- Added a selftest.
- Added a man page.
- Left open_exec() signature untouched to reduce patch impact
elsewhere (as suggested by Al Viro).
- Filled in bprm->filename with d_path() into a buffer, to avoid use
of potentially-ephemeral dentry->d_name.
- Patch against v3.14 (455c6fdbd21916).

David Drysdale (2):
syscalls,x86: implement execveat() system call
syscalls,x86: add selftest for execveat(2)

arch/x86/ia32/audit.c | 1 +
arch/x86/ia32/ia32entry.S | 1 +
arch/x86/kernel/audit_64.c | 1 +
arch/x86/kernel/entry_64.S | 28 ++++
arch/x86/syscalls/syscall_32.tbl | 1 +
arch/x86/syscalls/syscall_64.tbl | 2 +
arch/x86/um/sys_call_table_64.c | 1 +
fs/exec.c | 153 ++++++++++++++++---
include/linux/compat.h | 3 +
include/linux/sched.h | 4 +
include/linux/syscalls.h | 4 +
include/uapi/asm-generic/unistd.h | 4 +-
kernel/sys_ni.c | 3 +
lib/audit.c | 3 +
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/exec/.gitignore | 6 +
tools/testing/selftests/exec/Makefile | 32 ++++
tools/testing/selftests/exec/execveat.c | 251 ++++++++++++++++++++++++++++++++
18 files changed, 476 insertions(+), 23 deletions(-)
create mode 100644 tools/testing/selftests/exec/.gitignore
create mode 100644 tools/testing/selftests/exec/Makefile
create mode 100644 tools/testing/selftests/exec/execveat.c

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