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Date: Tue, 18 Feb 2020 22:44:47 -0500
From: Rich Felker <dalias@...c.org>
To: Zhang Tianci <zhangtianci1@...wei.com>
Cc: musl@...ts.openwall.com, yunlong.song@...wei.com
Subject: Re: [PATCH] stdio: Fix fdopen bug
On Wed, Feb 19, 2020 at 10:37:29AM +0800, Zhang Tianci wrote:
> Currently, in musl the fdopen doesn't check the consistence between
> fd's mode and corresponding file's mode.
>
> For example,
>
> int fd = open("file1", O_RDONLY);
> FILE *f = fdopen(fd, "W")
>
> In musl, above code will be Okay.
> While according to POSIX, above code (fdopen) will return EINVAL.
>
> Signed-off-by: Zhang Tianci <zhangtianci1@...wei.com>
> ---
> src/stdio/__fdopen.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/src/stdio/__fdopen.c b/src/stdio/__fdopen.c
> index 116e78e..23c4ffd 100644
> --- a/src/stdio/__fdopen.c
> +++ b/src/stdio/__fdopen.c
> @@ -26,6 +26,16 @@ FILE *__fdopen(int fd, const char *mode)
> /* Impose mode restrictions */
> if (!strchr(mode, '+')) f->flags = (*mode == 'r') ? F_NOWR : F_NORD;
>
> + int fd_flag = __syscall(SYS_fcntl, fd, F_GETFL);
> +
> + if (fd_flag == -1) return 0;
> +
> + if (((fd_flag & O_ACCMODE) == O_RDONLY && !(f->flags & F_NORD)) ||
> + ((fd_flag & O_ACCMODE) == O_WRONLY && !(f->flags & F_NOWR))) {
> + errno = EINVAL;
> + return 0;
> + }
> +
> /* Apply close-on-exec flag */
> if (strchr(mode, 'e')) __syscall(SYS_fcntl, fd, F_SETFD, FD_CLOEXEC);
>
> --
> 2.17.1
Per POSIX this is a "may fail" not a "shall fail". Testing for this is
more costly (see added code/syscalls in the patch) and serves no
purpose, which is why it's not done.
Rich
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