Ákos Uzonyi, 2020.08.24.
When strace is tracing a process which is in an other process namespace than strace itself, PIDs in system call arguments and return values are printed as PIDs in the target process’ namespace. This is normal behavior, as these are the actual values that are passed between the process and the kernel. But it would be useful to have an option for strace to translate these ids to strace’s own pid namespace. For example when tracing a process inside a Docker container, the translated PIDs allows us, to access these the processes from the host environment (eg. inspecting processes created by the clone syscall). The goal of the project is to add a flag to strace, which enables this translation.
Introduced a new --pidns-translation option, which makes strace print all PIDs in its own PID namespace too, like this:
getpid() = 4 /* 1238378 in strace's PID NS */
Also, various strace features which require reading files in /proc (thread enumeration, file descriptor decoding, mmap cache) are now working even when /proc is mounted from a different namespace (PIDs are translated for /proc/<pid> paths)
The reason this seemingly simple feature took a whole summer to implement, is that the Linux kernel does not provide an interface for translating PIDs between namespaces. The approach used in this implementation is reading all the /proc/<pid>/status files, and finding the PID to be translated in the “NSpid:” line.
The implementation of this feature is done in 5 commits:
PID namespace translation support
Adds the main file responsible for the translation (pidns.c), a trie implementation, the –pidns-translation option, and a description to the manual.
Use printpid in decoders
Modifies syscall decoders, to print PIDs using the printpid function in pidns.c.
Use get_proc_pid for /proc paths
Where a /proc/<pid> directory is accessed, adds a get_proc_pid call to translate the PID to the namespace of /proc.
Implement testing framework for pidns
Adds a little framework for pidns tests (tests/pidns.c), which creates child processes in a new PID namespace, and helps printing the expected translation PID.
Add tests for PID namespace translation
Adds an extra test for each decoder modified in “Use printpid in decoders”. These tests are run in a new PID namespace (created by the framework in the previous commit).
Commits on github
(All commits listed here are made during GSoC 2020, although not all of them is related to the main project)