On Tue, Jun 16, 2020 at 03:14:44PM +0200, Vegard Nossum wrote: > > On 2020-06-05 22:19, [email protected] wrote: >> The patch titled >> Subject: exec: open code copy_string_kernel >> has been removed from the -mm tree. Its filename was >> exec-open-code-copy_string_kernel.patch >> >> This patch was dropped because it was merged into mainline or a subsystem >> tree >> >> ------------------------------------------------------ >> From: Christoph Hellwig <[email protected]> >> Subject: exec: open code copy_string_kernel >> >> Currently copy_string_kernel is just a wrapper around copy_strings that >> simplifies the calling conventions and uses set_fs to allow passing a >> kernel pointer. But due to the fact the we only need to handle a single >> kernel argument pointer, the logic can be sigificantly simplified while >> getting rid of the set_fs. >> >> Link: http://lkml.kernel.org/r/[email protected] >> Signed-off-by: Christoph Hellwig <[email protected]> >> Cc: Alexander Viro <[email protected]> >> Signed-off-by: Andrew Morton <[email protected]> >> --- >> >> fs/exec.c | 45 +++++++++++++++++++++++++++++++++++---------- >> 1 file changed, 35 insertions(+), 10 deletions(-) >> >> --- a/fs/exec.c~exec-open-code-copy_string_kernel >> +++ a/fs/exec.c >> @@ -592,17 +592,42 @@ out: >> */ >> int copy_string_kernel(const char *arg, struct linux_binprm *bprm) >> { >> - int r; >> - mm_segment_t oldfs = get_fs(); >> - struct user_arg_ptr argv = { >> - .ptr.native = (const char __user *const __user *)&arg, >> - }; >> - >> - set_fs(KERNEL_DS); >> - r = copy_strings(1, argv, bprm); >> - set_fs(oldfs); >> + int len = strnlen(arg, MAX_ARG_STRLEN) + 1 /* terminating NUL */; >> + unsigned long pos = bprm->p; >> - return r; >> + if (len == 0) >> + return -EFAULT; > > Just a quick question, how can len ever be 0 here when len was set to > strnlen() + 1? Should the test be different? > > The old version (i.e. copy_strings()) seems to return -EFAULT when > strnlen() returns 0.
So, the nasty part here is that strnlen_user has different semantics from strnlen: - strlen excludes the terminating null byte and never returns error codes - strnlen_user includes the terminating null byte, and a 0 return means it could not access the user address (a condition that can't happen for strlen). Now with that back to your original question: I think then len == 0 check can just be removed without replacement.
