Re: [patch V4 07/12] uaccess: Provide scoped user access regions

David Laight
2025-10-22T14:20:10Z

On Wed, 22 Oct 2025 14:49:10 +0200 (CEST)
Thomas Gleixner <tglx@linutronix.de> wrote:

> User space access regions are tedious and require similar code patterns all
> over the place:
>
> if (!user_read_access_begin(from, sizeof(*from)))
> return -EFAULT;
> unsafe_get_user(val, from, Efault);
> user_read_access_end();
> return 0;
> Efault:
> user_read_access_end();
> return -EFAULT;
>
> This got worse with the recent addition of masked user access, which
> optimizes the speculation prevention:
>
> if (can_do_masked_user_access())
> from = masked_user_read_access_begin((from));
> else if (!user_read_access_begin(from, sizeof(*from)))
> return -EFAULT;
> unsafe_get_user(val, from, Efault);
> user_read_access_end();
> return 0;
> Efault:
> user_read_access_end();
> return -EFAULT;
>
> There have been issues with using the wrong user_*_access_end() variant in
> the error path and other typical Copy&Pasta problems, e.g. using the wrong
> fault label in the user accessor which ends up using the wrong accesss end
> variant.
>
> These patterns beg for scopes with automatic cleanup. The resulting outcome
> is:
> scoped_user_read_access(from, Efault)
> unsafe_get_user(val, from, Efault);
> return 0;
> Efault:
> return -EFAULT;
>
> The scope guarantees the proper cleanup for the access mode is invoked both
> in the success and the failure (fault) path.
>
> The scoped_user_$MODE_access() macros are implemented as self terminating
> nested for() loops. Thanks to Andrew Cooper for pointing me at them. The
> scope can therefore be left with 'break', 'goto' and 'return'. Even
> 'continue' "works" due to the self termination mechanism.

I think that 'feature' should be marked as a 'bug', consider code like:
for (; len >= sizeof (*uaddr); uaddr++; len -= sizeof (*uaddr)) {
scoped_user_read_access(uaddr, Efault) {
int frag_len;
unsafe_get_user(frag_len, &uaddr->len, Efault);
if (!frag_len)
break;
...
}
...
}

The expectation would be that the 'break' applies to the visible 'for' loop.
But you need a 'goto' to escape from the visible loop.

Someone who groks the static checkers might want to try to detect
continue/break in those loops.

David