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Subject: bug#79684: [PATCH] Check native comp ABI version
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Content-Type: text/plain

Tags: patch

Native-comp currenty does not explicitly check that the ABI version when
an .eln was produced is the same as the ABI version when an .eln is
loaded. It relies on putting a substring of an MD5 sum in the file name
instead (see hash_native_abi).

This proved insufficient in a case I had here, which is why I added an
explicit check to my Emacs.

Andrea is in CC.


--=-=-=
Content-Type: text/x-patch
Content-Disposition: attachment;
 filename=0001-Check-native-comp-ABI-version.patch

From d1bb66339a63769c9596eb1f902d30105ba8ac0a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Gerd=20M=C3=B6llmann?= <gerd.moellmann@HIDDEN>
Date: Thu, 23 Oct 2025 14:21:32 +0200
Subject: [PATCH] Check native comp ABI version

* src/comp.c (ABI_VERSION): Make an integer.
(ABI_VERSION_SYMBOL): New.
(hash_native_abi): Convert ABI_VERSION to string.
(emit_abi_version, check_abi_version): New functions.
(Fcomp__compile_ctxt_to_file0): Emit ABI_VERSION_SYMBOL.
(load_comp_unit): Check ABI version.
---
 src/comp.c | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/src/comp.c b/src/comp.c
index 8ab167ab592..a487fa7dbe5 100644
--- a/src/comp.c
+++ b/src/comp.c
@@ -468,11 +468,16 @@ load_gccjit_if_necessary (bool mandatory)
 
 
 /* Increase this number to force a new Vcomp_abi_hash to be generated.  */
-#define ABI_VERSION "12"
+#define ABI_VERSION 13
 
 /* Length of the hashes used for eln file naming.  */
 #define HASH_LENGTH 8
 
+/* The name of a global emitted to the text segment that contains the
+   ABI version that was used to generate the file.  This is checked
+   against the current ABI version when a file is loaded.  */
+#define ABI_VERSION_SYM STR (ABI_ ## ABI_VERSION)
+
 /* C symbols emitted for the load relocation mechanism.  */
 #define CURRENT_THREAD_RELOC_SYM "current_thread_reloc"
 #define F_SYMBOLS_WITH_POS_ENABLED_RELOC_SYM "f_symbols_with_pos_enabled_reloc"
@@ -784,7 +789,7 @@ hash_native_abi (void)
 
   Vcomp_abi_hash =
     comp_hash_string (
-      concat3 (build_string (ABI_VERSION),
+      concat3 (build_string (STR (ABI_VERSION)),
 	       concat3 (Vemacs_version, Vsystem_configuration,
 			Vsystem_configuration_options),
 	       Fmapconcat (intern_c_string ("comp--subr-signature"),
@@ -2959,6 +2964,28 @@ #define ADD_IMPORTED(f_name, ret_type, nargs, args)			       \
   return Freverse (field_list);
 }
 
+/* Emit an exported global whose symbol name contains the ABI
+   version used when generating it.  */
+
+static void
+emit_abi_version (void)
+{
+  gcc_jit_context_new_global (comp.ctxt, NULL, GCC_JIT_GLOBAL_EXPORTED,
+			      comp.void_ptr_type, ABI_VERSION_SYM);
+}
+
+/* Check ABI version of CU against the current version. Do this because
+   relying on a substring of an MD5 checksum as part of an eln's file
+   name is prone to fail.  */
+
+static void
+check_abi_version (struct Lisp_Native_Comp_Unit *cu)
+{
+  if (dynlib_sym (cu->handle, ABI_VERSION_SYM) == NULL)
+    error ("File '%s' has incompatible ABI version",
+	   SDATA (cu->file));
+}
+
 /*
   This emit the code needed by every compilation unit to be loaded.
 */
@@ -4840,6 +4867,7 @@ DEFUN ("comp--compile-ctxt-to-file0", Fcomp__compile_ctxt_to_file0,
     CALLNI (comp-data-container-idx, CALLNI (comp-ctxt-d-ephemeral, Vcomp_ctxt));
 
   emit_ctxt_code ();
+  emit_abi_version ();
 
   /* Define inline functions.  */
   define_CAR_CDR ();
@@ -5255,6 +5283,8 @@ load_comp_unit (struct Lisp_Native_Comp_Unit *comp_u, bool loading_dump,
 
   if (!comp_u->loaded_once)
     {
+      check_abi_version (comp_u);
+
       struct thread_state ***current_thread_reloc =
 	dynlib_sym (handle, CURRENT_THREAD_RELOC_SYM);
       bool **f_symbols_with_pos_enabled_reloc =
-- 
2.51.1


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Subject: bug#79684: Acknowledgement ([PATCH] Check native comp ABI version)
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Subject: bug#79684: [PATCH] Check native comp ABI version
Resent-From: Andrea Corallo <acorallo@HIDDEN>
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From: Andrea Corallo <acorallo@HIDDEN>
In-Reply-To: <m2o6px4wdo.fsf@HIDDEN> ("Gerd =?UTF-8?Q?M=C3=B6llmann?="'s message of "Thu, 23 Oct 2025 14:48:19 +0200")
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Gerd M=C3=B6llmann <gerd.moellmann@HIDDEN> writes:

> Tags: patch
>
> Native-comp currenty does not explicitly check that the ABI version when
> an .eln was produced is the same as the ABI version when an .eln is
> loaded. It relies on putting a substring of an MD5 sum in the file name
> instead (see hash_native_abi).
>
> This proved insufficient in a case I had here, which is why I added an
> explicit check to my Emacs.
>
> Andrea is in CC.

Hi Gerd,

could you describe exactly which scenario proved the current setup to be
insufficient?  AFAIR we use the the ABI version to form the filename to
look for no?

Thanks!

  Andrea

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Subject: bug#79684: [PATCH] Check native comp ABI version
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Andrea Corallo <acorallo@HIDDEN> writes:

> Gerd M=C3=B6llmann <gerd.moellmann@HIDDEN> writes:
>
>> Tags: patch
>>
>> Native-comp currenty does not explicitly check that the ABI version when
>> an .eln was produced is the same as the ABI version when an .eln is
>> loaded. It relies on putting a substring of an MD5 sum in the file name
>> instead (see hash_native_abi).
>>
>> This proved insufficient in a case I had here, which is why I added an
>> explicit check to my Emacs.
>>
>> Andrea is in CC.
>
> Hi Gerd,
>
> could you describe exactly which scenario proved the current setup to be
> insufficient?  AFAIR we use the the ABI version to form the filename to
> look for no?

Not 100% exactly I'm afraid, but let me try. It went something like
this:

I have a "very" old todo item that I'm working on and off when I feel
like it. This is about the roots igc is using to scan the Lisp_Objects
in, mainly, the d_relocs vectors in the data segments of .eln files.
They have to be scanned because MPS is a copying collector and objects
can move in memory. These roots are in sum pretty large, in my case,
with my init file and so on, they amount to ca. 1.5 MB, which increases
GC latency because all roots must be scanned before anything can be
copied during GC and so on.

These d_reloc vectors are filled when an .eln is loaded by using the
Lisp reader to produce a vector from a string the text segment. The
contents of this vector are then copied to the d_reloca arrays in the
data segment.

The idea I'm trying out, with very slow progress, is to change this to
use an additional indirection in the native code. I'm not copying the
vector's contents to the data segment d_reloc, but store a pointer to
the "contents" member of the vector. That is, where the code currently
does something akin to

  static Lisp_Object d_reloc[42];
  ...
  Lisp_Object x =3D d_reloc[17];

it then does the moral equivalent of=20

  static Lisp_Object **d_reloc;
  ...
  Lisp_Object x =3D (*d_reloc)[17];

IOW, I'm trying to trade less GC latency for an additional indirection,
and see which one I like better.
=20=20
For all that I changed ABI_VERSION. Then I built, and it loaded an .eln
that had the wrong version, which took me a looong time to make sense
of. That's basically the background of the whole story (plus that there
seems to also be something weird in the build system that sometimes
leads to loading stuff from an installation in ~/.local, but that's a
different story.)

Anyway, when I checked this

comp.c:
  802 void
  803 hash_native_abi (void)
  804 {
  805   /* Check runs once.  */
  806   eassert (NILP (Vcomp_abi_hash));
  807=20
  808   Vcomp_abi_hash =3D
  809     comp_hash_string (
  810       concat3 (build_string (STR (ABI_VERSION)),
  811                concat3 (Vemacs_version, Vsystem_configuration,
  812                         Vsystem_configuration_options),
  813                Fmapconcat (intern_c_string ("comp--subr-signature"),
  814                            Vcomp_subr_list, build_string (""))));
  815=20

I had my doubts that each change in only the ABI_VERSION for example
lands in the result of that function. MD5 and substring, and so on. The
patch caught that case, FWIW.

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Gerd M=C3=B6llmann <gerd.moellmann@HIDDEN> writes:

> Andrea Corallo <acorallo@HIDDEN> writes:
>
>> Gerd M=C3=B6llmann <gerd.moellmann@HIDDEN> writes:
>>
>>> Tags: patch
>>>
>>> Native-comp currenty does not explicitly check that the ABI version when
>>> an .eln was produced is the same as the ABI version when an .eln is
>>> loaded. It relies on putting a substring of an MD5 sum in the file name
>>> instead (see hash_native_abi).
>>>
>>> This proved insufficient in a case I had here, which is why I added an
>>> explicit check to my Emacs.
>>>
>>> Andrea is in CC.
>>
>> Hi Gerd,
>>
>> could you describe exactly which scenario proved the current setup to be
>> insufficient?  AFAIR we use the the ABI version to form the filename to
>> look for no?
>
> Not 100% exactly I'm afraid, but let me try. It went something like
> this:
>
> I have a "very" old todo item that I'm working on and off when I feel
> like it. This is about the roots igc is using to scan the Lisp_Objects
> in, mainly, the d_relocs vectors in the data segments of .eln files.
> They have to be scanned because MPS is a copying collector and objects
> can move in memory. These roots are in sum pretty large, in my case,
> with my init file and so on, they amount to ca. 1.5 MB, which increases
> GC latency because all roots must be scanned before anything can be
> copied during GC and so on.
>
> These d_reloc vectors are filled when an .eln is loaded by using the
> Lisp reader to produce a vector from a string the text segment. The
> contents of this vector are then copied to the d_reloca arrays in the
> data segment.
>
> The idea I'm trying out, with very slow progress, is to change this to
> use an additional indirection in the native code. I'm not copying the
> vector's contents to the data segment d_reloc, but store a pointer to
> the "contents" member of the vector. That is, where the code currently
> does something akin to
>
>   static Lisp_Object d_reloc[42];
>   ...
>   Lisp_Object x =3D d_reloc[17];
>
> it then does the moral equivalent of=20
>
>   static Lisp_Object **d_reloc;
>   ...
>   Lisp_Object x =3D (*d_reloc)[17];
>
> IOW, I'm trying to trade less GC latency for an additional indirection,
> and see which one I like better.
>=20=20=20
> For all that I changed ABI_VERSION. Then I built, and it loaded an .eln
> that had the wrong version, which took me a looong time to make sense
> of. That's basically the background of the whole story (plus that there
> seems to also be something weird in the build system that sometimes
> leads to loading stuff from an installation in ~/.local, but that's a
> different story.)
>
> Anyway, when I checked this
>
> comp.c:
>   802 void
>   803 hash_native_abi (void)
>   804 {
>   805   /* Check runs once.  */
>   806   eassert (NILP (Vcomp_abi_hash));
>   807=20
>   808   Vcomp_abi_hash =3D
>   809     comp_hash_string (
>   810       concat3 (build_string (STR (ABI_VERSION)),
>   811                concat3 (Vemacs_version, Vsystem_configuration,
>   812                         Vsystem_configuration_options),
>   813                Fmapconcat (intern_c_string ("comp--subr-signature"),
>   814                            Vcomp_subr_list, build_string (""))));
>   815=20
>
> I had my doubts that each change in only the ABI_VERSION for example
> lands in the result of that function. MD5 and substring, and so on. The
> patch caught that case, FWIW.

I'm closing this because it's part of another patch to igc that I posted
on emacs-devel.

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From: =?utf-8?Q?Gerd_M=C3=B6llmann?= <gerd.moellmann@HIDDEN>
Subject: control message for bug #79684
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close 79684 31.1
quit

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