fix: enforce permission checks for allow_other mounts

[ShukantPal]

Closes libfuse#15

When default_permissions is not set and allow_other is enabled, the kernel's fuse_permission() returns 0 for most permission checks without consulting userspace. This means that after a chmod 000, cached dentries still allow access by other users — the kernel never sends FUSE_ACCESS to verify permissions.

This is a two-layer problem requiring fixes in both the kernel and the FUSE daemon:

Daemon fix (example/passthrough.c):
Fix xmp_access() to check permissions as the requesting user (via fuse_get_context()) rather than as root. Uses raw SYS_setresuid/SYS_setresgid syscalls for per-thread credential switching, since glibc wrappers synchronize across all threads.

Associated kernel patch (fs/fuse/dir.c fuse_permission):
When allow_other is set and default_permissions is not, add an else-if branch that sends FUSE_ACCESS to userspace for all permission checks. Includes a MAY_NOT_BLOCK guard that returns -ECHILD to force retry outside RCU walk mode, since fuse_access() sends a blocking request to userspace.

--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1607,6 +1607,17 @@ static int fuse_permission(...)
      } else if (mask & (MAY_ACCESS | MAY_CHDIR)) {
          err = fuse_access(inode, mask);
+     } else if (fc->allow_other) {
+         /*
+          * With allow_other, multiple users can access this mount.
+          * Verify permissions via userspace to prevent user A from
+          * accessing files/directories owned by user B with
+          * restrictive permissions.
+          */
+         if (mask & MAY_NOT_BLOCK)
+             return -ECHILD;
+
+         err = fuse_access(inode, mask);
      } else if ((mask & MAY_EXEC) && S_ISREG(inode->i_mode)) {

Tests (xfail without kernel patch):

• test_perm_cache_dir_traversal: chmod 000 dir, verify files inside are denied
• test_perm_cache_file_truncate: chmod 000 file, verify truncate is denied
• test_perm_cache_dir_read_after_noexec: remove dir execute bit, verify traversal is denied

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