Intercept build commands using ptrace

[i-ky]

Some time ago I was in a search for a way to generate compile_commands.json for a project with a very tricky build system that did not respect CC/CXX, used hardcoded compiler paths and statically compiled binaries here and there. Unfortunately, bear is not able to intercept commands in this case. Eventually I came across compile-db-gen which uses strace and then developed my own solution based on ptrace. I wonder if you would be interested in adding such command interception mechanism to bear. If yes, I will be glad to help.

[rizsotto]

Hey @i-ky , this is a good proposal... Bear is already using 2 interception mode (the compiler wrappers and the library preload), adding another one would increase the chance to cover more builds. And Bear is already split the work of recognizing compiler calls and intercepting process executions.

Was looking at your project, it looks very simple. I would be glad to review PRs on this work, if you have time. (I won't be able to work on this in the near future, because I am traveling at the moment.)

[rizsotto]

Thanks for the explanation @i-ky . As a follow up question, is this a linux specific solution only? Or can work with other OS too? (The wiki page mentions that ptrace itself is available on other platforms too. But wondering if the /proc/[pid]/* files are standard locations across OSes?)

[i-ky]

As a follow up question, is this a linux specific solution only?

Yes, /proc/[pid]/{exe,cwd,cmdline} seem to be Linux-only.

[i-ky]

Pure-ptrace solution would support more OSes, but it is way more complicated, because it would need to track chdir() syscalls and this is not a trivial task since ptrace only provides syscall numbers which vary depending on the architecture. Building on top of strace would be an option, but strace/strace#70 or strace/strace#169 needs to be implemented first.

[rizsotto]

I was looking into run strace and parse its output, but failed to get the full argument list of the function calls.

[i-ky]

compile-db-gen uses -s/--string-limit option and passes the result of getconf ARG_MAX as a value, see here. This tactic works, but compile-db-gen has a couple of other problems. Not to mention, that parsing strace output with regular expressions is neither efficient nor robust.