Re: [RFC PATCH] fs: allow open(dir, O_TMPFILE|..., 0) with mode 0

[Andy Lutomirski]

On 10/30/2014 02:08 AM, Eric Rannaud wrote:
> The man page for open(2) indicates that when O_CREAT is specified, the
> 'mode' argument applies only to future accesses to the file:
> 
> 	Note that this mode applies only to future accesses of the newly
> 	created file; the open() call that creates a read-only file
> 	may well return a read/write file descriptor.
> 
> The man page for open(2) implies that 'mode' is treated identically by
> O_CREAT and O_TMPFILE.
> 
> O_TMPFILE, however, behaves differently:
> 
> 	int fd = open("/tmp", O_TMPFILE | O_RDWR, 0);
> 	assert(fd == -1);
> 	assert(errno == EACCES);
> 
> 	int fd = open("/tmp", O_TMPFILE | O_RDWR, 0600);
> 	assert(fd > 0);
> 
> For O_CREAT, do_last() sets acc_mode to MAY_OPEN only:
> 
> 	if (*opened & FILE_CREATED) {
> 		/* Don't check for write permission, don't truncate */
> 		open_flag &= ~O_TRUNC;
> 		will_truncate = false;
> 		acc_mode = MAY_OPEN;
> 		path_to_nameidata(path, nd);
> 		goto finish_open_created;
> 	}
> 
> But for O_TMPFILE, do_tmpfile() passes the full op->acc_mode to
> may_open().
> 
> This patch lines up the behavior of O_TMPFILE with O_CREAT. After the
> inode is created, may_open() is called with acc_mode = MAY_OPEN, in
> do_tmpfile().
> 
> A different, but related glibc bug revealed the discrepancy:
> https://sourceware.org/bugzilla/show_bug.cgi?id=17523
> 
> The glibc lazily loads the 'mode' argument of open() and openat() using
> va_arg() only if O_CREAT is present in 'flags' (to support both the 2
> argument and the 3 argument forms of open; same idea for openat()).
> However, the glibc ignores the 'mode' argument if O_TMPFILE is in
> 'flags'.
> 
> On x86_64, for open(), it magically works anyway, as 'mode' is in
> RDX when entering open(), and is still in RDX on SYSCALL, which is where
> the kernel looks for the 3rd argument of a syscall.
> 
> But openat() is not quite so lucky: 'mode' is in RCX when entering the
> glibc wrapper for openat(), while the kernel looks for the 4th argument
> of a syscall in R10. Indeed, the syscall calling convention differs from
> the regular calling convention in this respect on x86_64. So the kernel
> sees mode = 0 when trying to use glibc openat() with O_TMPFILE, and
> fails with EACCES.

Looks sensible.  Should this be Cc: stable?

--Andy

> 
> Signed-off-by: Eric Rannaud <e@xxxxxxxxxxxxxxxx>
> ---
>  fs/namei.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 42df664e95e5..78512898d3ba 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -3154,7 +3154,8 @@ static int do_tmpfile(int dfd, struct filename *pathname,
>  	if (error)
>  		goto out2;
>  	audit_inode(pathname, nd->path.dentry, 0);
> -	error = may_open(&nd->path, op->acc_mode, op->open_flag);
> +	/* Don't check for other permissions, the inode was just created */
> +	error = may_open(&nd->path, MAY_OPEN, op->open_flag);
>  	if (error)
>  		goto out2;
>  	file->f_path.mnt = nd->path.mnt;
> 

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