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prctl: add PR_GET_AUXV to copy auxv to userspace

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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>
This commit is contained in:
Josh Triplett 2023-04-04 21:31:48 +09:00 committed by Andrew Morton
parent 97f7e09481
commit ddc65971bb
2 changed files with 17 additions and 0 deletions
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2
include/uapi/linux/prctl.h
View File

@ -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 */

15
kernel/sys.c
View File

@ -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;
}