Be careful with errno

I’m getting close to releasing version 3.3.11 of procps.  When it gets near that time, I generally browse again the Debian Bug Tracker for procps bugs. Bug number #733758 caught my eye.  With the free command if you used the s option before the c option, the s option failed, “seconds argument ‘N’ failed” where N was the number you typed in. The error should be for you trying to type letters for number of seconds. Seemed reasonably simple to test and simple to fix.

Take me to the code

The relevant code looks like this:

   case 's':
            flags |= FREE_REPEAT;
            args.repeat_interval = (1000000 * strtof(optarg, &endptr));
            if (errno || optarg == endptr || (endptr && *endptr))
                xerrx(EXIT_FAILURE, _("seconds argument `%s' failed"), optarg);

Seems pretty stock-standard sort of function. Use strtof() to convert the string into the float.

You need to check both errno AND optarg == endptr because:

  • A valid but large float means errno = ERANGE
  • A invalid float (e.g. “FOO”) means optarg == endptr

At first I thought the logic was wrong, but tracing through it was fine.  I then compiled free using the upstream git source, the program worked fine with s flag with no c flag. Doing a diff between the upstream HEAD and Debian’s 3.3.10 source showed nothing obvious.

I then shifted the upstream git to 3.3.10 too and re-compiled. The Debian source failed, the upstream parsed the s flag fine. I ran diff, no change. I ran md5sum, the hashes matched; what is going on here?

I’ll set when I want

The man page says in the case of under/overflow “ERANGE is stored in errno”. What this means is if there isn’t and under/overflow then errno is NOT set to 0, but its just not set at all. This is quite useful when you have a chain of functions and you just want to know something failed, but don’t care what.

Most of the time, you generally would have a “Have I failed?” test and then check errno for why. A typical example is socket calls where anything less than 0 means failure. You check the return value first and then errno. strtof() is one of those funny ones where most people check errno directly; its simpler than checking for +/- HUGE_VAL. You can see though that there are traps.

What’s the difference?

OK, so a simple errno=0 above the call fixes it, but why would the Debian source tree have this failure and the upstream not? Even with the same code? The difference is how they are compiled.

The upstream compiles free like this:

gcc -std=gnu99 -DHAVE_CONFIG_H -I. -include ./config.h -I./include -DLOCALEDIR=\"/usr/local/share/locale\" -Iproc -g -O2 -MT free.o -MD -MP -MF .deps/free.Tpo -c -o free.o free.c
mv -f .deps/free.Tpo .deps/free.Po
/bin/bash ./libtool --tag=CC --mode=link gcc -std=gnu99 -Iproc -g -O2 ./proc/ -o free free.o strutils.o fileutils.o -ldl
libtool: link: gcc -std=gnu99 -Iproc -g -O2 -o .libs/free free.o strutils.o fileutils.o ./proc/.libs/ -ldl


While Debian has some hardening flags:

gcc -std=gnu99 -DHAVE_CONFIG_H -I. -include ./config.h -I./include -DLOCALEDIR=\"/usr/share/locale\" -D_FORTIFY_SOURCE=2 -Iproc -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -MT free.o -MD -MP -MF .deps/free.Tpo -c -o free.o free.c
mv -f .deps/free.Tpo .deps/free.Po
/bin/bash ./libtool --tag=CC --mode=link gcc -std=gnu99 -Iproc -g -O2 -fstack-protector-strong -Wformat -Werror=format-security ./proc/ -Wl,-z,relro -o free free.o strutils.o fileutils.o -ldl
libtool: link: gcc -std=gnu99 -Iproc -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -Wl,-z -Wl,relro -o .libs/free free.o strutils.o fileutils.o ./proc/.libs/ -ldl

It’s not the compiling of free itself that is doing it, but the library. Most likely something that is called before the strtof() is setting errno which this code then falls into. In fact if you run the upstream free linked to the Debian procps library it fails.

Moral of the story is to set errno before the function is called if you are going to depend on it for checking if the function succeeded.


Linux 4.0 ate my docker images

I have previously written about the gitlab CI runners that use docker.  Yesterday I made some changes to procps and pushed them to gitlab which would then start the CI.  This morning I checked and it said build failed – ok, so that’s not terribly unusual. The output from the runner was:

gitlab-ci-multi-runner 0.3.3 (dbaf96f)
Using Docker executor with image csmall/testdebian ...
Pulling docker image csmall/testdebian ...
Build failed with Error: image csmall/testdebian: not found

Hmm, I know I have that image, it just must be the runner so, let’s see what images I have:

$ docker images

Now, I know I have images, I had about 10 or so of them, where did they go? I even looked in the /var/lib/docker directories and can see the json configs, what have you done with my images docker?

Read more Linux 4.0 ate my docker images

Debian, WordPress and Multi-site

For quite some time, the Debian version of WordPress has had a configuration tweak that made it possible to run multiple websites on the same server. This came from a while ago when multi-site wasn’t available. While a useful feature, it does make the initial setup of WordPress for simple sites more complicated.

I’m looking at changing the Debian package slightly so that for a single-site use it Just Works. I have also looked into the way WordPress handles the content, especially themes and plugins, to see if there is a way of updating them through the website itself. This probably won’t suit everyone but I think its a better default.

Read more Debian, WordPress and Multi-site

procps using GitLab CI

procps-ciThe procps project for a few years has been hosted at Gitorious.  With the announcement that Gitorious has been acquired by GitLab and that all repositories need to move there, procps moved along to GitLab. At first I thought it would just be a like for like thing, but then I noticed that GitLab has this GitLab CI feature and had to try it out.

CI here stands for Continuous Integration and is a way of automatically testing your program builds using a bunch of test scripts.  procps already has a set of tests, with some a level of coverage that has room for improvement, so it was a good candidate to use for CI. The way GitLab works is they have a central control point that is linked to the git repo and you create runners, which are the systems that actually compile the programs and run the tests. The runners then feed back their results and GitLab CI shows it all in pretty red or green.

Read more procps using GitLab CI

Backporting and git-buildpackage

For working with Debian packages, one method of maintaining them is to put them in git and use git-buildpackage to build them right out of the git repository.  There are a few pitfalls with it, notably around if you forget to import the upstream you get this strange treeish related error which still throws me at first when I see it.

Part of maintaining packages is to be able to fix security bugs in older versions of them that are found in stable and even sometimes old stable (jessie and wheezy respectively at the time of writing).  At first I used to do this outside git because to me there wasn’t a clear way of doing it within it.  This is not too satisfactory because it means you lose the benefits of using git in the first place, and for distributions you are more likely to need collaboration with, such as working with the security team or help with backporting.

Read more Backporting and git-buildpackage