On Tue, Apr 21, 2020 at 10:19:49PM -0700, Josh Triplett wrote:
> Some applications want to prevent the usual "lowest available fd"
> allocation from allocating certain file descriptors. For instance, they
> may want to prevent allocation of a closed fd 0, 1, or 2 other than via
> dup2/dup3, or reserve some low file descriptors for other purposes.
>
> Add a prctl to increase the minimum fd and return the previous minimum.
>
> System calls that allocate a specific file descriptor, such as
> dup2/dup3, ignore this minimum.
>
> exec resets the minimum fd, to prevent one program from interfering with
> another program's expectations about fd allocation.
Please make this aspect properly documented in "Effect on process
attributes" section of execve(2) manual page.
[...]
> +unsigned int increase_min_fd(unsigned int num)
> +{
> + struct files_struct *files = current->files;
> + unsigned int old_min_fd;
> +
> + spin_lock(&files->file_lock);
> + old_min_fd = files->min_fd;
> + files->min_fd += num;
> + spin_unlock(&files->file_lock);
> + return old_min_fd;
> +}
If it's "increase", there should be an overflow check.
Otherwise it's "assign" rather than "increase".
--
ldv
