RE: [PATCH 3/4] exec: simplify the compat syscall handling

From: Al Viro

Sent: 26 March 2021 16:12

On Fri, Mar 26, 2021 at 03:38:30PM +0100, Christoph Hellwig wrote:

quoted

+static const char __user *
+get_user_arg_ptr(const char __user *const __user *argv, int nr)
 {
+	if (in_compat_syscall()) {
+		const compat_uptr_t __user *compat_argv =
+			compat_ptr((unsigned long)argv);
 		compat_uptr_t compat;

+		if (get_user(compat, compat_argv + nr))
 			return ERR_PTR(-EFAULT);
 		return compat_ptr(compat);
+	} else {
+		const char __user *native;

+		if (get_user(native, argv + nr))
+			return ERR_PTR(-EFAULT);
+		return native;
+	}
 }

Yecchhh....  So you have in_compat_syscall() called again and again, for
each argument in the list?  I agree that current version is fucking ugly,
but I really hate that approach ;-/

Especially since in_compat_syscall() isn't entirely trivial on x86-64.
Probably all in the noise for 'exec', but all the bits do add up.

You may not want separate get_user() on some architectures either.
The user_access_begin/end aren't cheap.

OTOH if you call copy_from_user() you get hit by the stupid
additional costs of 'user copy hardening'.

	David

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