prctl: add PR_GET_AUXV to copy auxv to userspace

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.

Link: https://lkml.kernel.org/r/d81864a7f7f43bca6afa2a09fc2e850e4050ab42.1680611394.git.josh@joshtriplett.org
Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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 */

kernel/sys.c

@@ -2388,6 +2388,16 @@ 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, unsigned long len)
+{
+	struct mm_struct *mm = current->mm;
+	unsigned long size = min_t(unsigned long, 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)
 {
@@ -2518,6 +2528,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			else
 				return -EINVAL;
 			break;
+	case PR_GET_AUXV:
+		if (arg4 || arg5)
+			return -EINVAL;
+		error = prctl_get_auxv((void __user *)arg2, arg3);
+		break;
 		default:
 			return -EINVAL;
 		}