使用libssl / libcrypto进行Segfault

Question

This is more of a hypothetical whilst I'm debugging some code. Lets say I have an application (called X) that calls out to a lib to send an email over a TLS encrypted SMTP connection, whilst at the same time X is talking to another lib which is establishing another TLS socket through the same libcrypto lib, what's the likelyhood of getting some specific (and weird) condition where one function call would fail with a segfault?

I'm kind of grasping at straws, this code worked fine up until we added the Skype SDK which connects over TLS to the Skype servers, since then we can actually get the issue to be repeatable but I'm a bit baffled as to why. (I'm probably overlooking the obvious, but I'll start with the really weird possibility)

Answer 1

Quite genrally speaking it could be possible - but well written library should be robust to multiple access. You might want to look through documentation to see if their API is reentrant (or even thred-safe).

If it is thread-safe, then (assuming that libcrypto authors didn't make mistake) you can be sure that it's not the reason of the problem.

If it is reentrant, then anything using this lib in two (or more) threads should be synchronized on access (eg. using mutexes), but if parts of code are not written by you and you have no option to modiffy it, then you are stuck. The only thing i can think of would be to use another version of libcrypto, so system creates another, unrelated instance of it's internal structure. This is ugly soultion and might behave weird on user machines.

Answer 2

There's a whole man page dedicated to the usage of the OpenSSL library and threads: man 3 threads . You will need to use this if your application has multiple threads that use the OpenSSL library.