Add 'get_helper_error_msg' utility

[jordalgo]

This is for additional helpful messaging for specific errors e.g. for when the number of bpf map entries is exceeded.

Follow up to: #2809

Sample output:

$ sudo BPFTRACE_MAP_KEYS_MAX=1 bpftrace -e 'BEGIN { @[1] = 1; @[2] = 2; exit() }' -k
Attaching 1 probe...
stdin:1:24-25: WARNING: Map full; can't update element. Try increasing MAP_KEYS_MAX.
Additional Info - helper: map_update_elem, retcode: -7
BEGIN { @[1] = 1; @[2] = 2; exit() }
                       ~
@[1]: 1

$ sudo BPFTRACE_MAP_KEYS_MAX=1 bpftrace -e 'BEGIN { @[1] = 1; @[2] = 2; exit() }' -k -f json
{"type": "attached_probes", "data": {"probes": 1}}
{"type": "helper_error", "msg": "Map full; can't update element. Try increasing MAP_KEYS_MAX.", "helper": "map_update_elem", "retcode": -7, "line": 1, "col": 24}

{"type": "map", "data": {"@": {"1": 1}}}

Checklist

• Language changes are updated in man/adoc/bpftrace.adoc and if needed in docs/reference_guide.md
• User-visible and non-trivial changes updated in CHANGELOG.md
• The new behaviour is covered by tests

[viktormalik]

Looks good! Do we know about other helpers that could use better error messages?

[jordalgo]

Do we know about other helpers that could use better error messages?

Good question. I'll take a look and see if anything jumps out.

[danobi]

nice!

[danobi]

Changelog entry?

[ajor]

Good addition! While we're adding this extra detail to this specific error message, I'm thinking that we might also want to remove unnecessary info from it too. i.e. "Failed to map_update_elem: Argument list too long"

map_update_elem is an implementation detail and "Argument list too long" doesn't mean much.

Maybe we should say something more user friendly along the lines of: "Map full, can't update element (try ...)"

[ajor]

Suggested change

	msg.append(" (trying increasing MAP_KEYS_MAX)");
	msg.append(" (try increasing MAP_KEYS_MAX)");

Typo

[jordalgo]

map_update_elem is an implementation detail and "Argument list too long" doesn't mean much.

Yeah good call. Part of me wants to leave in 'map_update_elem' for devs debugging the issue even though it's an implementation detail. The json output separates this bpf function into a field called "helper". Maybe for non-json output we can just follow the error message with a similar format e.g.

Map full; can't update element. Try increasing MAP_KEYS_MAX. Additional Info - helper: map_update_elem, retcode: -7

[jordalgo]

Do we know about other helpers that could use better error messages?

Added two more for map lookup and deletion.

[danobi]

This is the "old" name for w/ the config changes. Sounds like there's consensus on changing the env var names - should we preemptively use BPFTRACE_MAX_MAP_KEYS here?

[jordalgo]

Depends on the merging order :) This is a smaller change so I imagine this will get merged first. Happy to change it with the config PR.

[danobi]

I think this is good to merge. Then the other PR can rebase on top

[viktormalik]

@ajor can we merge this?

[ajor]

Looks good to me!