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

From: Corey Bryant
Subject: Re: [Qemu-devel] [PATCH 2/3] block: Add support to "open" /dev/fd/X filenames
Date: Mon, 04 Jun 2012 12:28:36 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20120430 Thunderbird/12.0.1

On 06/04/2012 12:03 PM, Eric Blake wrote:
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

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 */

I don't think this is legitimate. errno can be set under the covers of library calls even if the strtol() call is successful.

I was thinking if strtol returns 0 and errno is 0, perhaps we could assume success, but I don't think this is guaranteed either.

Maybe a combination of isdigit() then strtol() will give a better idea of success.

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.



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