Commit 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and
optionally clear info about PTEs") made do_pagemap_cmd() the ioctl
handler of /proc/<pid>/pagemap.

However, do_pagemap_cmd() returns -EINVAL for every command other than
PAGEMAP_SCAN. Before that commit, pagemap had no ioctl handler, and
the VFS returned -ENOTTY for these commands. As of now, they fail with
EINVAL, so tcgetattr() and isatty() on pagemap report EINVAL instead
of ENOTTY.

The issue can be reproduced with a simple C program:

    #include <fcntl.h>
    #include <stdio.h>
    #include <termios.h>

    int main(void)
    {
            struct termios t;
            int fd = open("/proc/self/pagemap", O_RDONLY);

            if (tcgetattr(fd, &t) < 0)
                    perror("tcgetattr");
            return 0;
    }

Before that commit, the result is:

    tcgetattr: Inappropriate ioctl for device

After that commit, the result is:

    tcgetattr: Invalid argument

This patch will return -ENOIOCTLCMD for unknown commands, as
procfs_procmap_ioctl() in the same file does, so that the VFS turns it
into -ENOTTY.

Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Cc: stable@vger.kernel.org
Signed-off-by: Hengyu Liang <hengyul@cs.unc.edu>
---
Unknown commands should fail with ENOTTY: Documentation/driver-api/
ioctl.rst says so and calls -EINVAL wrong, and POSIX requires
tcgetattr() to fail with ENOTTY on a file that is not a terminal.

I found no user space that relies on EINVAL here: Wine and the mm and
kvm selftests only use PAGEMAP_SCAN on pagemap, whose results do not
change, and CRIU and LTP only read the file.

 fs/proc/task_mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 5c54aebe2118..92b275dc0d59 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -3307,7 +3307,7 @@ static long do_pagemap_cmd(struct file *file, unsigned int cmd,
 		return do_pagemap_scan(mm, arg);
 
 	default:
-		return -EINVAL;
+		return -ENOIOCTLCMD;
 	}
 }
 

base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b
-- 
2.53.0