libbpf-tools/filetop,offcputime: Fix stack leaks including KASLR break

[vdasu]

Description

libbpf-tools/filetop: get_file_path() copies the dentry name with bpf_probe_read_kernel(buf, size, dname.name) where size = PATH_MAX (4096). The non-_str probe read helper copies the full size with no NUL stop, so a short name over-reads ~4 KB of adjacent kernel memory into the file_stat.filename map value. I confirmed this leaks kernel .text and module pointers (e.g. kernfs_dops, ext4_file_operations) into the entries map, and I was able to recover the KASLR slide. Fix: use bpf_probe_read_kernel_str(), which stops at the NUL.

libbpf-tools/offcputime: handle_sched_switch() leaves struct val_t val uninitialized, then bpf_probe_read_kernel_str(&val.comm, ...) writes only strlen+1 bytes before the whole val is inserted into the info map. For short comms the val.comm tail keeps stale BPF-stack bytes (the adjacent start_ts timestamp), now persisted in the map. Same uninitialized-stack class as the bashreadline fix (c2d8f9b). Fix: zero-initialize the struct (struct val_t *valp, val = {};).

Why this approach

Minimal and consistent with the tree: bpf_probe_read_kernel_str() is the idiomatic kernel-string read (e.g. hardirqs), and zeroing the struct follows the bashreadline precedent. Neither changes behaviour for well-formed inputs.

---

Checklist

• Commit prefix matches changed area (libbpf-tools/filetop:, libbpf-tools/offcputime:)
• Commit body explains why this change is needed

For new tools only — N/A (bug fixes to existing tools, no new tool)

• Explains why this tool is needed and what existing tools cannot cover this use case
• Includes at least one real production use case
• Man page (man/man8/) with an OVERHEAD section
• Example output file (*_example.txt)
• README.md entry added
• Smoke test added to tests/python/test_tools_smoke.py

[ekyooo]

1. The two checklist items are about the commit message. Please improve it, including the typo.
2. Please squash the two commits.

[vdasu]

@ekyooo I have fixed the typo and updated the commit message