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Re: [Qemu-devel] [PATCH 2/3] block: Add support to "open" /dev/fd/X file

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 2/3] block: Add support to "open" /dev/fd/X filenames
Date: Mon, 04 Jun 2012 10:03:33 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1

On 06/04/2012 09:51 AM, Corey Bryant wrote:

>>> +
>>> +    if (strstart(filename, "/dev/fd/",&p)) {
>>> +        fd = atoi(p);
>> atoi() is lousy - it has no error checking, and returns 0 if a mistake
>> was made.  You really want to be using strtol (or even better, a
>> sensible wrapper around strtol that takes care of the subtleties of
>> calling it correctly), so that you don't end up dup'ing stdin when the
>> user passes a bad /dev/fd/ string.
> It looks like strtol returns 0 on failure too.  Do we need to support
> stdin/stdout/stderr?

But at least strtol lets you detect errors:

char *tmp;
errno = 0;
fd = strtol(p, &tmp, 10);
if (errno || tmp == p) {
    /* raise your error here */

and if you get past that point, then someone really did pass in
/dev/fd/0 as the string they meant to be parsed (probably a user bug, as
getfd is unlikely to ever return 0 unless you start with stdin closed,
which itself is something that POSIX discourages, but not something we
need to specifically worry about).  So I would argue that yes, we do
need to support fd 0, if only by not special casing it as compared to
any other valid fd.

Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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