tools/bashreadline: auto-detect libreadline.so for dynamically linked bash

[xuchunmei000]

On systems where /bin/bash dynamically links libreadline (which is the common case on modern distros), the readline symbol in the bash ELF is merely an undefined import (SHN_UNDEF, st_value=0). BCC's attach_uretprobe then fails with:

Exception: could not determine address of symbol readline in /bin/bash

The tool already provides a -s flag for users to manually specify the libreadline.so path, but this is inconvenient and breaks automated smoke tests.

This patch adds automatic detection: after resolving the symbol name, it checks whether the symbol is actually defined (not just imported) in the target binary. If not, it uses ldd /bin/bash to find the real libreadline.so path and redirects the uprobe there.

The logic is backward-compatible:

• If bash statically links readline -> symbol is defined, no change
• If user passes -s -> manual path used, no auto-detection
• If bash dynamically links readline -> auto-fallback to .so

Description

Why this approach

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Checklist

• Commit prefix matches changed area (e.g., tools/toolname:, libbpf-tools/toolname:, src/cc:, docs:, build:, tests/python:)
• Commit body explains why this change is needed

For new tools only

• 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

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