Re: [PATCH] vfs: add faccessat2 syscall
From: Florian Weimer <hidden>
Date: 2020-04-16 14:44:07
Also in:
linux-fsdevel, lkml
* Miklos Szeredi:
POSIX defines faccessat() as having a fourth "flags" argument, while the linux syscall doesn't have it. Glibc tries to emulate AT_EACCESS and AT_SYMLINK_NOFOLLOW, but AT_EACCESS emulation is broken. Add a new faccessat(2) syscall with the added flags argument and implement both flags. The value of AT_EACCESS is defined in glibc headers to be the same as AT_REMOVEDIR. Use this value for the kernel interface as well, together with the explanatory comment.
Thanks a lot for this!
+long do_faccessat(int dfd, const char __user *filename, int mode, int flags)
+{
+ const struct cred *old_cred = NULL;
+ struct path path;
+ struct inode *inode;
+ int res;
+ unsigned int lookup_flags = LOOKUP_FOLLOW;
+
+ if (mode & ~S_IRWXO) /* where's F_OK, X_OK, W_OK, R_OK? */
+ return -EINVAL;
+
+ if (flags & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW))
+ return -EINVAL;Should this accept AT_EMPTY_PATH as well? (I can't comment on the rest of the logic of the patch.)
quoted hunk
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h index ca88b7bce553..2f86b2ad6d7e 100644 --- a/include/uapi/linux/fcntl.h +++ b/include/uapi/linux/fcntl.h@@ -84,10 +84,20 @@ #define DN_ATTRIB 0x00000020 /* File changed attibutes */ #define DN_MULTISHOT 0x80000000 /* Don't remove notifier */ +/* + * The constants AT_REMOVEDIR and AT_EACCESS have the same value. AT_EACCESS is + * meaningful only to faccessat, while AT_REMOVEDIR is meaningful only to + * unlinkat. The two functions do completely different things and therefore, + * the flags can be allowed to overlap. For example, passing AT_REMOVEDIR to + * faccessat would be undefined behavior and thus treating it equivalent to + * AT_EACCESS is valid undefined behavior. + */ #define AT_FDCWD -100 /* Special value used to indicate openat should use the current working directory. */ #define AT_SYMLINK_NOFOLLOW 0x100 /* Do not follow symbolic links. */ +#define AT_EACCESS 0x200 /* Test access permitted for + effective IDs, not real IDs. */ #define AT_REMOVEDIR 0x200 /* Remove directory instead of unlinking file. */ #define AT_SYMLINK_FOLLOW 0x400 /* Follow symbolic links. */
I can confirm that this is what glibc does, ofr better or worse. Thanks, Florian