Caring about stack usage

It may not be surprising that there’s been a few projects over the years that I’ve worked on where we’ve had to care about stack usage (to varying degrees).

For threaded userspace applications (e.g. MySQL, Drizzle) you get a certain amount of stack per thread – and you really don’t want to bust that. For a great many years now, there’s been both a configuration parameter in MySQL to set how much stack each thread (connection) gets as well as various checks in the source code to ensure there’s enough free stack to do a particular operation (IIRC open_table is the most hairy one of this in MySQL).

For the Linux Kernel, stack usage is a relatively (in)famous problem… although by now just about every real problem has been fixed and merely mentioning it is probably just the influence of the odd grey beard hairs I’m pretending not to notice.

In a current project I’m working on, it’s also something we have to care about.

It turns out that GCC has a few nice things to help you prevent unbounded stack usage or runaway stack usage. There’s two warnings you can enable.

There’s -Wstack-usage=len which will throw warnings on unbounded stack usage (e.g. array on stack sized based on an argument to the function), where stack usage is greater than len and when stack usage may exceed len.

There’s also -Wframe-larger-than=len which is based on calculation for a particular stack frame, as opposed to -Wstack-usage=len, which could be based on several stack frames.

Odds are, you may get some warnings in your project if you set this to what you would consider “conservative” values. Now, if this is every going to explode at runtime is something that’s left as an exercise for the reader, but enabling these warnings is pretty easy and a simple way to help find and prevent some issues.

After all, having your software explode for running off the end of the stack is just a tad embarrassing.

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