[DTrace-devel] [PATCH v2] dtprobed: use /proc/$pid/map_files, not the filename of the mapping

[Eugene Loh]

On 2/12/26 13:43, Kris Van Hees wrote:

> I believe the following on top of the original patch should resolve this:
>
> diff --git a/dtprobed/dtprobed.c b/dtprobed/dtprobed.c
> index 9a692805..34349deb 100644
> --- a/dtprobed/dtprobed.c
> +++ b/dtprobed/dtprobed.c
> @@ -487,16 +487,17 @@ handle_usdt_notes(pid_t pid, uintptr_t addr)
>   		fuse_log(FUSE_LOG_ERR, "%i: dtprobed: cannot look up mapping (process dead?)\n",
>   			 pid);
>   		goto out;
> -	} else if ((fn = Pmap_mapfile_name(P, mapp)) == NULL) {
> +	} else if (prf->prf_mapname == NULL ||
> +		   (fn = Pmap_mapfile_name(P, mapp)) == NULL) {
>   		fuse_log(FUSE_LOG_ERR, "%i: dtprobed: cannot look up mapname (process dead?)\n",
>   			 pid);
>   		goto out;
>   	}
> -	mod = strrchr(fn, '/');
> +	mod = strrchr(prf->prf_mapname, '/');
>   	if (mod)
>   		mod++;
>   	else
> -		mod = fn;

Is that a '-' on the last line?

> On Thu, Feb 12, 2026 at 01:37:06PM -0500, Kris Van Hees wrote:
>> Oh, easy solution.... determine the mod name from prf->prf_mapname like we
>> did before.  I.e. work with two filenames, one from prf->prf_mapname to get
>> the module name, and one from Pmap_mapfile_name() to access the mapping data.
>>
>> On Thu, Feb 12, 2026 at 01:28:08PM -0500, Kris Van Hees via DTrace-devel wrote:
>>> Here is the problem:
>>>
>>>          prf = mapp->pr_file;
>>>          if (prf == NULL || (mapp = prf->first_segment) == NULL) {
>>>                  fuse_log(FUSE_LOG_ERR, "%i: dtprobed: cannot look up mapping (process dead?)\n",
>>>                           pid);
>>>                  goto out;
>>>          } else if ((fn = Pmap_mapfile_name(P, mapp)) == NULL) {
>>>                  fuse_log(FUSE_LOG_ERR, "%i: dtprobed: cannot look up mapname (process dead?)\n",
>>>                           pid);
>>>                  goto out;
>>>          }
>>>          mod = strrchr(fn, '/');
>>>          if (mod)
>>>                  mod++;
>>>          else
>>>                  mod = fn;
>>>
>>> Since you changed this to get the mapping filename in map_files, you get a
>>> name that has the address range as its filename, and since that gets used as
>>> the module name, you end up with the wrong module name in the probe spec.
>>>
>>> It needs to be the actual filename of the source file that the mapping came
>>> from.
>>>
>>> On Thu, Feb 12, 2026 at 06:18:25PM +0000, Nick Alcock wrote:
>>>> On 12 Feb 2026, Eugene Loh outgrape:
>>>>
>>>>> I do not know what went wrong here.  I would have bet big money that I
>>>>> tested this, but here is what I find now with this patch:
>>>> So did I!
>>>>
>>>>> test/unittest/usdt/tst.enabled.sh: Running timeout --signal=TERM 41 test/unittest/usdt/tst.enabled.sh /home/.../build/dtrace
>>>>> FAIL: expected results differ.
>>>>> 400000-401000:main:go
>>>>>
>>>>> Diff against expected:
>>>>> --- test/unittest/usdt/tst.enabled.r    2026-02-12 05:31:24.000000000 +0000
>>>>> +++ /tmp/runtest.6196/test.out  2026-02-12 18:09:58.980466976 +0000
>>>>> @@ -1,2 +1,2 @@
>>>>> -test:main:go
>>>>> +400000-401000:main:go
>>>> This suggests that symbol lookup is failing, which it shouldn't be,
>>>> because that *too* uses /proc/$pid/map_files, or should.
>>>>
>>>> I clearly need to debug this. Got a system on which it goes wrong?
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