[PATCH/RFC] powerpc/module_64: allow .init_array constructors to run

gcov and kasan rely on compiler generated constructor code.
For modules, gcc-8 with gcov enabled generates .init_array section,
but on ppc64le it doesn't get executed. find_module_sections() never
finds .init_array section, because module_frob_arch_sections() renames
it to _init_array.

Avoid renaming .init_array section, so do_mod_ctors() can use it.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Cc: Christophe Leroy <redacted>
Signed-off-by: Jan Stancek <jstancek@redhat.com>
---
I wasn't able to trace the comment:
  "We don't handle .init for the moment: rename to _init"
to original patch (it pre-dates .git). I'm not sure if it
still applies today, so I limited patch to .init_array. This
fixes gcov for modules for me on ppc64le 5.14.0-rc6.

Renaming issue is also mentioned in kasan patches here:
  https://patchwork.ozlabs.org/project/linuxppc-dev/cover/20210319144058.772525-1-dja@axtens

 arch/powerpc/kernel/module_64.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 6baa676e7cb6..c604b13ea6bf 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c

@@ -299,8 +299,16 @@ int module_frob_arch_sections(Elf64_Ehdr *hdr,
 					  sechdrs[i].sh_size);
 
 		/* We don't handle .init for the moment: rename to _init */
-		while ((p = strstr(secstrings + sechdrs[i].sh_name, ".init")))
+		while ((p = strstr(secstrings + sechdrs[i].sh_name, ".init"))) {
+#ifdef CONFIG_CONSTRUCTORS
+			/* find_module_sections() needs .init_array intact */
+			if (strstr(secstrings + sechdrs[i].sh_name,
+				".init_array")) {
+				break;
+			}
+#endif
 			p[0] = '_';
+		}
 
 		if (sechdrs[i].sh_type == SHT_SYMTAB)
 			dedotify((void *)hdr + sechdrs[i].sh_offset,

-- 
2.27.0

Le 17/08/2021 à 15:02, Jan Stancek a écrit :

gcov and kasan rely on compiler generated constructor code.
For modules, gcc-8 with gcov enabled generates .init_array section,
but on ppc64le it doesn't get executed. find_module_sections() never
finds .init_array section, because module_frob_arch_sections() renames
it to _init_array.

Avoid renaming .init_array section, so do_mod_ctors() can use it.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Cc: Christophe Leroy <redacted>
Signed-off-by: Jan Stancek <jstancek@redhat.com>
---
I wasn't able to trace the comment:
   "We don't handle .init for the moment: rename to _init"

Original patch there: https://github.com/mpe/linux-fullhistory/commit/d6ad6690aa72

Please avoid #ifdefery as much as possible.

I think here you can do:

			if (IS_ENABLED(CONFIG_CONSTRUCTORS) &&
			    strstr(secstrings + sechdrs[i].sh_name, ".init_array"))

+			/* find_module_sections() needs .init_array intact */
+			if (strstr(secstrings + sechdrs[i].sh_name,
+				".init_array")) {

No { } for single statements (See 
https://www.kernel.org/doc/html/latest/process/coding-style.html#placing-braces-and-spaces)

+				break;
+			}
+#endif
  			p[0] = '_';
+		}
  
  		if (sechdrs[i].sh_type == SHT_SYMTAB)
  			dedotify((void *)hdr + sechdrs[i].sh_offset,

Jan Stancek [off-list ref] writes:

gcov and kasan rely on compiler generated constructor code.
For modules, gcc-8 with gcov enabled generates .init_array section,
but on ppc64le it doesn't get executed. find_module_sections() never
finds .init_array section, because module_frob_arch_sections() renames
it to _init_array.

Avoid renaming .init_array section, so do_mod_ctors() can use it.

This (the existing renaming) breaks a KASAN test too, so I'd love to see
this fixed.

However, I don't know that renaming the section is a complete fix: from
memory it is still be possible to get relocations that are too far away
and require separate trampolines. But I wasn't able to construct a
module that exhibited this behaviour and test a fix before I got pulled
away to other things.

Kind regards,
Daniel

Le 17/08/2021 à 15:02, Jan Stancek a écrit :

gcov and kasan rely on compiler generated constructor code.
For modules, gcc-8 with gcov enabled generates .init_array section,
but on ppc64le it doesn't get executed. find_module_sections() never
finds .init_array section, because module_frob_arch_sections() renames
it to _init_array.

Avoid renaming .init_array section, so do_mod_ctors() can use it.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Cc: Christophe Leroy <redacted>
Signed-off-by: Jan Stancek <jstancek@redhat.com>

Does commit d4be60fe66b7 ("powerpc/module_64: use module_init_section 
instead of patching names") fixes your issue ?

If not, please rebase and resubmit.

Thanks
Christophe

On Thu, Jul 7, 2022 at 1:20 PM Christophe Leroy
[off-list ref] wrote:

Le 17/08/2021 à 15:02, Jan Stancek a écrit :

quoted

gcov and kasan rely on compiler generated constructor code.
For modules, gcc-8 with gcov enabled generates .init_array section,
but on ppc64le it doesn't get executed. find_module_sections() never
finds .init_array section, because module_frob_arch_sections() renames
it to _init_array.

Avoid renaming .init_array section, so do_mod_ctors() can use it.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Cc: Christophe Leroy <redacted>
Signed-off-by: Jan Stancek <jstancek@redhat.com>

Does commit d4be60fe66b7 ("powerpc/module_64: use module_init_section
instead of patching names") fixes your issue ?

Yes, it does gcov for me. Thanks

If not, please rebase and resubmit.

Thanks
Christophe

quoted

---
I wasn't able to trace the comment:
   "We don't handle .init for the moment: rename to _init"
to original patch (it pre-dates .git). I'm not sure if it
still applies today, so I limited patch to .init_array. This
fixes gcov for modules for me on ppc64le 5.14.0-rc6.

Renaming issue is also mentioned in kasan patches here:
   https://patchwork.ozlabs.org/project/linuxppc-dev/cover/20210319144058.772525-1-dja@axtens

  arch/powerpc/kernel/module_64.c | 10 +++++++++-
  1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 6baa676e7cb6..c604b13ea6bf 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c

@@ -299,8 +299,16 @@ int module_frob_arch_sections(Elf64_Ehdr *hdr,
                                        sechdrs[i].sh_size);

              /* We don't handle .init for the moment: rename to _init */
-             while ((p = strstr(secstrings + sechdrs[i].sh_name, ".init")))
+             while ((p = strstr(secstrings + sechdrs[i].sh_name, ".init"))) {
+#ifdef CONFIG_CONSTRUCTORS
+                     /* find_module_sections() needs .init_array intact */
+                     if (strstr(secstrings + sechdrs[i].sh_name,
+                             ".init_array")) {
+                             break;
+                     }
+#endif
                      p[0] = '_';
+             }

              if (sechdrs[i].sh_type == SHT_SYMTAB)
                      dedotify((void *)hdr + sechdrs[i].sh_offset,