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

[Kris Van Hees]

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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