src/sss_client/common.c: Use getpwnam_r to avoid clobbering struct passwd

[salahcoronya]

If something else uses PAM (like openrc, see
OpenRC/openrc#984) and getpwnam, and calls something like pam_open_session, sssd's call to getpwnam in init_sssd_ids clobbers the cached value by the other program.

[gemini-code-assist]

Code Review

This pull request correctly replaces the non-thread-safe getpwnam with its re-entrant version, getpwnam_r, to prevent potential data corruption when multiple threads are involved. The implementation follows the standard pattern for using getpwnam_r, including dynamic buffer allocation and resizing. However, I've found a potential memory leak in the error handling of realloc and provided a suggestion to fix it.

[alexey-tikhonov]

/gemini review

[gemini-code-assist]

Code Review

This pull request correctly replaces the non-reentrant getpwnam with the reentrant getpwnam_r to prevent thread-safety issues. The implementation for handling ERANGE is mostly correct, but it has a potential denial-of-service vulnerability due to unbounded memory allocation. I've added a review comment with a suggested fix to address this by adding a size limit to the buffer.

[alexey-tikhonov]

Hi @salahcoronya,

I think this reports a valid bug, thanks.

But there is a number of (at least) stylistic issues (re SSSD coding style) with the patch.
Would you prefer to finalize the patch yourself (and then I'll provide review comments) or do you prefer somebody from maintainers take over?

[salahcoronya]

I went ahead and finalized it myself, follow the coding style document.