[PATCH] cifs: ignore FL_FLOCK locks in read/write

["Aurélien Aptel" <aaptel@suse.com>]

From: Aurelien Aptel <aaptel@suse.com>

flock(2)-type locks are advisory, they are not supposed to prevent IO
if mode would otherwise allow it. From man page:

   flock()  places  advisory  locks  only; given suitable permissions on a
   file, a process is free to ignore the use of flock() and perform I/O on
   the file.

Simple reproducer:

	#include <stdlib.h>
	#include <stdio.h>
	#include <errno.h>
	#include <sys/file.h>
	#include <sys/types.h>
	#include <sys/wait.h>
	#include <unistd.h>

	int main(int argc, char** argv)
	{
		const char* fn = argv[1] ? argv[1] : "aaa";
		int fd, status, rc;
		pid_t pid;

		fd = open(fn, O_RDWR|O_CREAT, S_IRWXU);
		pid = fork();

		if (pid == 0) {
			flock(fd, LOCK_SH);
			exit(0);
		}

		waitpid(pid, &status, 0);
		rc = write(fd, "xxx\n", 4);
		if (rc < 0) {
			perror("write");
			return 1;
		}

		puts("ok");
		return 0;
	}

If the locks are advisory the write() call is supposed to work
otherwise we are trying to write with only a read lock (aka shared
lock) so it fails.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
---
 fs/cifs/file.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 6d001905c8e5..3e351a534720 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -3242,6 +3242,7 @@ cifs_writev(struct kiocb *iocb, struct iov_iter *from)
 	struct inode *inode = file->f_mapping->host;
 	struct cifsInodeInfo *cinode = CIFS_I(inode);
 	struct TCP_Server_Info *server = tlink_tcon(cfile->tlink)->ses->server;
+	struct cifsLockInfo *lock;
 	ssize_t rc;
 
 	inode_lock(inode);
@@ -3257,7 +3258,7 @@ cifs_writev(struct kiocb *iocb, struct iov_iter *from)
 
 	if (!cifs_find_lock_conflict(cfile, iocb->ki_pos, iov_iter_count(from),
 				     server->vals->exclusive_lock_type, 0,
-				     NULL, CIFS_WRITE_OP))
+				     &lock, CIFS_WRITE_OP) || (lock->flags & FL_FLOCK))
 		rc = __generic_file_write_iter(iocb, from);
 	else
 		rc = -EACCES;
@@ -3975,6 +3976,7 @@ cifs_strict_readv(struct kiocb *iocb, struct iov_iter *to)
 	struct cifsFileInfo *cfile = (struct cifsFileInfo *)
 						iocb->ki_filp->private_data;
 	struct cifs_tcon *tcon = tlink_tcon(cfile->tlink);
+	struct cifsLockInfo *lock;
 	int rc = -EACCES;
 
 	/*
@@ -4000,7 +4002,7 @@ cifs_strict_readv(struct kiocb *iocb, struct iov_iter *to)
 	down_read(&cinode->lock_sem);
 	if (!cifs_find_lock_conflict(cfile, iocb->ki_pos, iov_iter_count(to),
 				     tcon->ses->server->vals->shared_lock_type,
-				     0, NULL, CIFS_READ_OP))
+				     0, &lock, CIFS_READ_OP) || (lock->flags & FL_FLOCK))
 		rc = generic_file_read_iter(iocb, to);
 	up_read(&cinode->lock_sem);
 	return rc;
-- 
2.30.0