Re: [PATCH v2] proc: alloc PATH_MAX bytes for /proc/${pid}/fd/ symlinks

Hao Lee
2022-01-23T14:23:50Z

On Sun, Jan 23, 2022 at 9:58 PM Alexey Dobriyan <adobriyan@gmail.com> wrote:
>
> From: Hao Lee <haolee.swjtu@gmail.com>
>
> It's not a standard approach that use __get_free_page() to alloc path
> buffer directly. We'd better use kmalloc and PATH_MAX.
>
> PAGE_SIZE is different on different archs.

This is indeed a worthy addition. Thanks.

Regards,
Hao Lee

> An unlinked file
> with very long canonical pathname will readlink differently
> because "(deleted)" eats into a buffer. --adobriyan
>
> Signed-off-by: Hao Lee <haolee.swjtu@gmail.com>
> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> ---
>
> fs/proc/base.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1764,25 +1764,25 @@ static const char *proc_pid_get_link(struct dentry *dentry,
>
> static int do_proc_readlink(struct path *path, char __user *buffer, int buflen)
> {
> - char *tmp = (char *)__get_free_page(GFP_KERNEL);
> + char *tmp = (char *)kmalloc(PATH_MAX, GFP_KERNEL);
> char *pathname;
> int len;
>
> if (!tmp)
> return -ENOMEM;
>
> - pathname = d_path(path, tmp, PAGE_SIZE);
> + pathname = d_path(path, tmp, PATH_MAX);
> len = PTR_ERR(pathname);
> if (IS_ERR(pathname))
> goto out;
> - len = tmp + PAGE_SIZE - 1 - pathname;
> + len = tmp + PATH_MAX - 1 - pathname;
>
> if (len > buflen)
> len = buflen;
> if (copy_to_user(buffer, pathname, len))
> len = -EFAULT;
> out:
> - free_page((unsigned long)tmp);
> + kfree(tmp);
> return len;
> }
>