syscall: check correct group in Faccessat

The Faccessat call checks the user, group, or other permission bits of a
file to see if the calling process can access it. The test to see if the
group permissions should be used was made with the wrong group id, using
the process's group id rather than the file's group id. Fix this to use
the correct group id.

No test since we cannot easily change file permissions when not running
as root and the test is meaningless if running as root.

For #52313

Change-Id: I4e2c84754b0af7830b40fd15dedcbc58374d75ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/399539
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: neild

src/syscall/syscall_linux.go

@@ -109,7 +109,7 @@ func Faccessat(dirfd int, path string, mode uint32, flags int) (err error) {
 			gid = Getgid()
 		}
 
-		if uint32(gid) == st.Gid || isGroupMember(gid) {
+		if uint32(gid) == st.Gid || isGroupMember(int(st.Gid)) {
 			fmode = (st.Mode >> 3) & 7
 		} else {
 			fmode = st.Mode & 7