Re: [PATCH] ggc, jit: forcibly clear GTY roots in jit

[Antoni Boucher <bouanto@zoho.com>]

It seems to not be enough to fix the issue.
Let me find out what's missing from my patch.

On Tue, 2023-09-12 at 11:35 +0200, Richard Biener via Jit wrote:
> On Wed, Sep 6, 2023 at 3:41 PM David Malcolm via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> > 
> > As part of Antoyo's work on supporting LTO in rustc_codegen_gcc, he
> > noticed an ICE inside libgccjit when compiling certain rust files.
> > 
> > Debugging libgccjit showed that outdated information from a
> > previous
> > in-memory compile was referring to ad-hoc locations in the previous
> > compile's line_table.
> > 
> > The issue turned out to be the function decls in
> > internal_fn_fnspec_array
> > from the previous compile keeping alive the symtab nodes for these
> > functions, and from this finding other functions in the previous
> > compile, walking their CFGs, and finding ad-hoc data pointers in an
> > edge
> > with a location_t using ad-hoc data from the previous line_table
> > instance, and thus a use-after-free ICE attempting to use this ad-
> > hoc
> > data.
> > 
> > Previously in toplev::finalize we've fixed global state "piecemeal"
> > by
> > calling out to individual source_name_cc_finalize functions. 
> > However,
> > it occurred to me that we have run-time information on where the
> > GTY-marked pointers are.
> > 
> > Hence this patch takes something of a "big hammer" approach by
> > adding a
> > new ggc_common_finalize that walks the GC roots, zeroing all of the
> > pointers.  I stepped through this in the debugger and observed
> > that, in
> > particular, this correctly zeroes the internal_fn_fnspec_array at
> > the end
> > of a libgccjit compile.  Antoyo reports that this fixes the ICE for
> > him.
> > Doing so uncovered an ICE with libgccjit in dwarf2cfi.cc due to
> > reuse of
> > global variables from the previous compile, which this patch also
> > fixes.
> > 
> > I noticed that in ggc_mark_roots when clearing deletable roots we
> > only
> > clear the initial element in each gcc_root_tab_t.  This looks like
> > a
> > latent bug to me, which the patch fixes.  That said, there don't
> > seem to
> > be any deletable roots where the number of elements != 1.
> > 
> > Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu.
> > 
> > OK for trunk?
> 
> OK.
> 
> Thanks,
> Richard.
> 
> > Thanks
> > Dave
> > 
> > gcc/ChangeLog:
> >         * dwarf2cfi.cc (dwarf2cfi_cc_finalize): New.
> >         * dwarf2out.h (dwarf2cfi_cc_finalize): New decl.
> >         * ggc-common.cc (ggc_mark_roots): Multiply by rti->nelt
> > when
> >         clearing the deletable gcc_root_tab_t.
> >         (ggc_common_finalize): New.
> >         * ggc.h (ggc_common_finalize): New decl.
> >         * toplev.cc (toplev::finalize): Call dwarf2cfi_cc_finalize
> > and
> >         ggc_common_finalize.
> > ---
> >  gcc/dwarf2cfi.cc  |  9 +++++++++
> >  gcc/dwarf2out.h   |  1 +
> >  gcc/ggc-common.cc | 23 ++++++++++++++++++++++-
> >  gcc/ggc.h         |  2 ++
> >  gcc/toplev.cc     |  3 +++
> >  5 files changed, 37 insertions(+), 1 deletion(-)
> > 
> > diff --git a/gcc/dwarf2cfi.cc b/gcc/dwarf2cfi.cc
> > index ddc728f4ad00..f1777c0a4cf1 100644
> > --- a/gcc/dwarf2cfi.cc
> > +++ b/gcc/dwarf2cfi.cc
> > @@ -3822,4 +3822,13 @@ make_pass_dwarf2_frame (gcc::context *ctxt)
> >    return new pass_dwarf2_frame (ctxt);
> >  }
> > 
> > +void dwarf2cfi_cc_finalize ()
> > +{
> > +  add_cfi_insn = NULL;
> > +  add_cfi_vec = NULL;
> > +  cur_trace = NULL;
> > +  cur_row = NULL;
> > +  cur_cfa = NULL;
> > +}
> > +
> >  #include "gt-dwarf2cfi.h"
> > diff --git a/gcc/dwarf2out.h b/gcc/dwarf2out.h
> > index 870b56a6a372..61a996050ff9 100644
> > --- a/gcc/dwarf2out.h
> > +++ b/gcc/dwarf2out.h
> > @@ -419,6 +419,7 @@ struct fixed_point_type_info
> >      } scale_factor;
> >  };
> > 
> > +void dwarf2cfi_cc_finalize (void);
> >  void dwarf2out_cc_finalize (void);
> > 
> >  /* Some DWARF internals are exposed for the needs of DWARF-based
> > debug
> > diff --git a/gcc/ggc-common.cc b/gcc/ggc-common.cc
> > index bed7a9d4d021..95803fa95a17 100644
> > --- a/gcc/ggc-common.cc
> > +++ b/gcc/ggc-common.cc
> > @@ -86,7 +86,7 @@ ggc_mark_roots (void)
> > 
> >    for (rt = gt_ggc_deletable_rtab; *rt; rt++)
> >      for (rti = *rt; rti->base != NULL; rti++)
> > -      memset (rti->base, 0, rti->stride);
> > +      memset (rti->base, 0, rti->stride * rti->nelt);
> > 
> >    for (rt = gt_ggc_rtab; *rt; rt++)
> >      ggc_mark_root_tab (*rt);
> > @@ -1293,3 +1293,24 @@ report_heap_memory_use ()
> >              SIZE_AMOUNT (MALLINFO_FN ().arena));
> >  #endif
> >  }
> > +
> > +/* Forcibly clear all GTY roots.  */
> > +
> > +void
> > +ggc_common_finalize ()
> > +{
> > +  const struct ggc_root_tab *const *rt;
> > +  const_ggc_root_tab_t rti;
> > +
> > +  for (rt = gt_ggc_deletable_rtab; *rt; rt++)
> > +    for (rti = *rt; rti->base != NULL; rti++)
> > +      memset (rti->base, 0, rti->stride * rti->nelt);
> > +
> > +  for (rt = gt_ggc_rtab; *rt; rt++)
> > +    for (rti = *rt; rti->base != NULL; rti++)
> > +      memset (rti->base, 0, rti->stride * rti->nelt);
> > +
> > +  for (rt = gt_pch_scalar_rtab; *rt; rt++)
> > +    for (rti = *rt; rti->base != NULL; rti++)
> > +      memset (rti->base, 0, rti->stride * rti->nelt);
> > +}
> > diff --git a/gcc/ggc.h b/gcc/ggc.h
> > index 34108e2f0061..3280314f8481 100644
> > --- a/gcc/ggc.h
> > +++ b/gcc/ggc.h
> > @@ -368,4 +368,6 @@ inline void gt_ggc_mx (unsigned long int) { }
> >  inline void gt_ggc_mx (long long int) { }
> >  inline void gt_ggc_mx (unsigned long long int) { }
> > 
> > +extern void ggc_common_finalize ();
> > +
> >  #endif
> > diff --git a/gcc/toplev.cc b/gcc/toplev.cc
> > index 6c1a6f443c16..db62e3e995ec 100644
> > --- a/gcc/toplev.cc
> > +++ b/gcc/toplev.cc
> > @@ -2336,6 +2336,7 @@ toplev::finalize (void)
> >    cgraph_cc_finalize ();
> >    cgraphunit_cc_finalize ();
> >    symtab_thunks_cc_finalize ();
> > +  dwarf2cfi_cc_finalize ();
> >    dwarf2out_cc_finalize ();
> >    gcse_cc_finalize ();
> >    ipa_cp_cc_finalize ();
> > @@ -2350,6 +2351,8 @@ toplev::finalize (void)
> >    save_decoded_options = NULL;
> >    save_decoded_options_count = 0;
> > 
> > +  ggc_common_finalize ();
> > +
> >    /* Clean up the context (and pass_manager etc). */
> >    delete g;
> >    g = NULL;
> > --
> > 2.26.3
> >