subprogram support causes problems accessing fentry args named 'fn'

[tyroguru]

What reproduces the bug? Provide code if possible.

On a 5.19 kernel with bpftrace HEAD commit == 742f0cf we get a naming clash when attempting to access an fentry argument named 'fn'.

$ sudo ./bpftrace -lv fentry:vmlinux:module_kallsyms_on_each_symbol
kfunc:vmlinux:module_kallsyms_on_each_symbol
    const char * modname
    int (*)(void *, const char *, struct module *, unsigned long) fn
    void * data
    int retval

$ sudo ./bpftrace -e 'fentry:vmlinux:module_kallsyms_on_each_symbol{ $t = args.fn; }'
stdin:1:53-60: ERROR: syntax error, unexpected subprog
fentry:vmlinux:module_kallsyms_on_each_symbol{ $t = args.fn; }
                                                    ~~~~~~~

@ajor suggested adding the SUBPROG token as a known keyword in the grammar as so:

diff --git a/src/parser.yy b/src/parser.yy
index 85c8a20e..89efca28 100644
--- a/src/parser.yy
+++ b/src/parser.yy
@@ -628,6 +628,7 @@ keyword:
         |       SIZEOF        { $$ = $1; }
         |       UNROLL        { $$ = $1; }
         |       WHILE         { $$ = $1; }
+        |       SUBPROG       { $$ = $1; }
         ;

The above modification makes bpftrace happier but whether it is a complete fix is another thing.

$ sudo ./bpftrace -e 'fentry:vmlinux:module_kallsyms_on_each_symbol{ $t = args.fn; }'
Attaching 1 probe...
ERROR: Error loading program: kfunc:vmlinux:module_kallsyms_on_each_symbol (try -v)

(The error here is a verifier problem and should be ignored for the purpose of this bug).

bpftrace --info output

[viktormalik]

cc @lenticularis39

[lenticularis39]

Oops, I planned to "do that later" for fn also (the addition to the grammar) and forgot, thanks for reporting that!

[lenticularis39]

fn will never appear in the place of an identifier, since subprog definitions are top-level so there is no problem with parsing it as a keyword.