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

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Date:   Sun, 23 Jan 2022 16:58:35 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     akpm@...ux-foundation.org
Cc:     christian.brauner@...ntu.com, keescook@...omium.org,
        jamorris@...ux.microsoft.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Hao Lee <haolee.swjtu@...il.com>
Subject: [PATCH v2] proc: alloc PATH_MAX bytes for /proc/${pid}/fd/ symlinks

From: Hao Lee <haolee.swjtu@...il.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. 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@...il.com>
Signed-off-by: Alexey Dobriyan <adobriyan@...il.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;
 }