[PATCH] fs/proc/task_mmu: return -ENOTTY for unknown pagemap ioctls

[PATCH] fs/proc/task_mmu: return -ENOTTY for unknown pagemap ioctls

[Hengyu Liang]

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

Re: [PATCH] fs/proc/task_mmu: return -ENOTTY for unknown pagemap ioctls

[Andrew Morton]

On Fri, 23 Oct 2026 14:13:00 -0400 Hengyu Liang <hengyul@cs.unc.edu> wrote:

> 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.

OK, thanks, annoying.

> Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")

Three years ago.  So if anyone had to update their userspace code
because we altered this, they will now need to unupdate it.  It's hard
to believe that this will cause significant harm - perhaps there are
correctness test suites which will be surprised (again).

> 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.

Good info, thanks.