Re: Re: Re: Re: [PATCH 1/3] switch_creds: Syscall to switch creds for file server ops

From: Tetsuo Handa
Date: Fri Nov 01 2013 - 09:24:59 EST

Jim Lieb wrote:
> Subsequent uses look like:
> use_creds(cached fd);
> followed by
> open/creat/mknod/write
> followed by
> use_creds(-1);

Are you aware that calling commit_creds() is prohibitted between
override_creds() and revert_creds() ?

If the caller does some operation that calls commit_creds() (like
example below), the kernel triggers BUG().

---------- example module start ----------
#include <linux/module.h>
#include <linux/cred.h>
#include <linux/fs.h>
#include <linux/file.h>

static int __init test_init(void)
{ /* switch_creds() syscall */
struct fd f = fdget(0);
if (!f.file)
return -EBADF;
{ /* something that calls commit_creds() */
struct cred *cred = prepare_creds();
if (cred)
return 0;

static void test_exit(void)

---------- example module end ----------

Since nobody can guarantee that the caller of switch_creds() never does
some operation that calls commit_creds(), I don't think switch_creds()
based on override_creds() will work.
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