Re: [PATCH] decode_stacktrace: Support heuristic caller address search

[Sasha Levin]

On Thu,  5 Mar 2026 14:12:19 +0900, Masami Hiramatsu (Google) wrote:
> Add -c option to search call address search to decode_stacktrace.
> This tries to decode line info backwards, starting from 1byte before
> the return address, and displays the first line info it founds as
> the caller address.
> If it tries up to 10bytes before (or the symbol address) and still
> can not find it, it gives up and decodes the return address.

The commit message says "up to 10bytes" but the code passes $offset
(the function offset from the symbol) as the max iteration count to
search_call_site(). There's no 10-byte cap anywhere in the code?
$offset can easily be hundreds or thousands of bytes into a function.

> +search_call_site() {
> +     # Instead of using the return address, use the nearest line info
> +     # address before given address.
> +     local return_addr=${2}
> +     local max=${3}
> +     local i
> +
> +     for i in $(seq 1 ${max}); do
> +             local expr=$((0x$return_addr-$i))
> +             local address=$(printf "%x\n" "$expr")
> +
> +             local code=$(${ADDR2LINE} -i -e "${1}" "$address" 2>/dev/null)
> +             local first=${code% *}
> +             if [[ "$code" != "" && "$code" != ${UNKNOWN_LINE} && "${first#*:}" != "?" ]]; then

To also address Matthieu's question about performance: I think this
whole iterative search could be replaced by simply subtracting 1 from
the return address before passing it to addr2line.

DWARF line tables map address *ranges* to source lines, so any address
within the CALL instruction resolves to the correct source line.
return_addr-1 is guaranteed to land inside the CALL instruction (it's
the last byte of it), so a single addr2line call is sufficient.

This is exactly what the kernel itself does in sprint_backtrace()
(kernel/kallsyms.c:570): it passes symbol_offset=-1 to
__sprint_symbol(), which does `address += symbol_offset` before
lookup. GDB, perf, and libunwind all use the same addr-1 trick for
the same reason.

That would make this both correct and free.

> +             if [[ "$code" != "" && "$code" != ${UNKNOWN_LINE} && "${first#*:}" != "?" ]]; then

Minor: ${UNKNOWN_LINE} is "??:0" -- when unquoted on the RHS of != inside
[[ ]], the ? characters are interpreted as glob wildcards (each matching
any single character). It happens to work here because ? also matches '?'
itself, but it should be quoted as "${UNKNOWN_LINE}" for correctness.
Same issue on the other != ${UNKNOWN_LINE} below.

--
Thanks,
Sasha