trace, argdist: Treat small USDT arguments correctly

trace and argdist currently only work correctly for USDT arguments
whose size is exactly 8 bytes. Smaller types, such as chars, shorts,
ints (signed or unsigned) are not treated correctly. The reason is
that the produced program would invoke the `bpf_usdt_readarg` helper
with the address of a u64 local variable, and then cast that variable
to the user-specified type derived from the format string. However,
the `bpf_usdt_readarg` rewriting then passes `sizeof(u64)` to the
generated `bpf_..._readarg` macro, which then fails to read anything
because the provided size doesn't match the argument size it knows
about.

The fix is fairly easy: instead of declaring a u64 unconditionally
and reading into that variable with `bpf_usdt_readarg`, declare a
variable that has the correct type according to what we know about
the USDT probe.

tools/argdist.py

@@ -293,9 +293,12 @@ def _generate_hash_field(self, i):
  def _generate_usdt_arg_assignment(self, i):
  expr = self.exprs[i]
  if self.probe_type == "u" and expr[0:3] == "arg":
- return (" u64 %s = 0;\n" +
+ arg_index = int(expr[3])
+ arg_ctype = self.usdt_ctx.get_probe_arg_ctype(
+ self.function, arg_index - 1)
+ return (" %s %s = 0;\n" +
  " bpf_usdt_readarg(%s, ctx, &%s);\n") \
- % (expr, expr[3], expr)
+ % (arg_ctype, expr, expr[3], expr)
  else:
  return ""
 

tools/trace.py

@@ -321,9 +321,12 @@ def _generate_field_assign(self, idx):
  expr = self.values[idx].strip()
  text = ""
  if self.probe_type == "u" and expr[0:3] == "arg":
- text = (" u64 %s = 0;\n" +
+ arg_index = int(expr[3])
+ arg_ctype = self.usdt.get_probe_arg_ctype(
+ self.usdt_name, arg_index - 1)
+ text = (" %s %s = 0;\n" +
  " bpf_usdt_readarg(%s, ctx, &%s);\n") \
- % (expr, expr[3], expr)
+ % (arg_ctype, expr, expr[3], expr)
 
  if field_type == "s":
  return text + """