[PATCH v3] prctl: Add PR_GET_AUXV to copy auxv to userspace

[Josh Triplett]

If a library wants to get information from auxv (for instance,
AT_HWCAP/AT_HWCAP2), it has a few options, none of them perfectly
reliable or ideal:

- Be main or the pre-main startup code, and grub through the stack above
  main. Doesn't work for a library.
- Call libc getauxval. Not ideal for libraries that are trying to be
  libc-independent and/or don't otherwise require anything from other
  libraries.
- Open and read /proc/self/auxv. Doesn't work for libraries that may run
  in arbitrarily constrained environments that may not have /proc
  mounted (e.g. libraries that might be used by an init program or a
  container setup tool).
- Assume you're on the main thread and still on the original stack, and
  try to walk the stack upwards, hoping to find auxv. Extremely bad
  idea.
- Ask the caller to pass auxv in for you. Not ideal for a user-friendly
  library, and then your caller may have the same problem.

Add a prctl that copies current->mm->saved_auxv to a userspace buffer.

Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
---

v2:
- Fix W=1 warning about min type mismatch
v3:
- Change argument type to size_t. (Left the return value as int to match
  all the other helpers in the file.)

I've built, booted, and tested this.

man-pages snippet:

.\" prctl PR_GET_AUXV
.TP
.BR PR_GET_AUXV " (since Linux 6.x)"
Get the auxilliary vector (auxv) into the buffer pointed to by
.IR "(void\~*) arg2" ,
whose length is given by \fIarg3\fP.
If the buffer is not long enough for the full auxilliary vector,
the copy will be truncated.
Return (as the function result)
the full length of the auxilliary vector.
\fIarg4\fP and \fIarg5\fP must be 0.

Will send a patch for man-pages once merged.

 include/uapi/linux/prctl.h |  2 ++
 kernel/sys.c               | 15 +++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 1312a137f7fb..b2b24eaf2427 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -290,4 +290,6 @@ struct prctl_mm_map {
 #define PR_SET_VMA		0x53564d41
 # define PR_SET_VMA_ANON_NAME		0
 
+#define PR_GET_AUXV		0x41555856
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 495cd87d9bf4..f031edb8bc47 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2377,6 +2377,16 @@ static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3,
 		PR_MDWE_REFUSE_EXEC_GAIN : 0;
 }
 
+static int prctl_get_auxv(void __user *addr, size_t len)
+{
+	struct mm_struct *mm = current->mm;
+	size_t size = min_t(size_t, sizeof(mm->saved_auxv), len);
+
+	if (size && copy_to_user(addr, mm->saved_auxv, size))
+		return -EFAULT;
+	return sizeof(mm->saved_auxv);
+}
+
 SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
 {
@@ -2661,6 +2671,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 	case PR_SET_VMA:
 		error = prctl_set_vma(arg2, arg3, arg4, arg5);
 		break;
+	case PR_GET_AUXV:
+		if (arg4 || arg5)
+			return -EINVAL;
+		error = prctl_get_auxv((void __user *)arg2, arg3);
+		break;
 	default:
 		error = -EINVAL;
 		break;
-- 
2.40.0