[v1] ARM: module: fix unwind section relocation out of range error

[PATCH] ARM: module: fix unwind section relocation out of range error

Posted by William Zhang 4 months, 2 weeks ago

In an armv7 system that uses non-3G/1G split and with more than 512MB
physical memory, driver load may fail with following error:
   section 29 reloc 0 sym '': relocation 42 out of range (0xc2ab9be8 ->
0x7fad5998)

This happens when relocation R_ARM_PREL31 from the unwind section
.ARM.extab and .ARM.exidx are allocated from the VMALLOC space while
.text section is from MODULES_VADDR space. It exceeds the +/-1GB
relocation requirement of R_ARM_PREL31 hence triggers the error.

The fix is to mark .ARM.extab and .ARM.exidx sections as executable so
they can be allocated within .text section and always meet range
requirement.

Co-developed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: William Zhang <william.zhang@broadcom.com>

---

 arch/arm/kernel/module-plts.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/arm/kernel/module-plts.c b/arch/arm/kernel/module-plts.c
index 354ce16d83cb..5f5bf5e63bd6 100644
--- a/arch/arm/kernel/module-plts.c
+++ b/arch/arm/kernel/module-plts.c
@@ -225,6 +225,18 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 			mod->arch.init.plt = s;
 		else if (s->sh_type == SHT_SYMTAB)
 			syms = (Elf32_Sym *)s->sh_addr;
+#if defined(CONFIG_ARM_UNWIND) && !defined(CONFIG_VMSPLIT_3G)
+		else if (s->sh_type == ELF_SECTION_UNWIND ||
+			 (strncmp(".ARM.extab", secstrings + s->sh_name, 10) == 0)) {
+			/*
+			 * To avoid the possible relocation out of range issue for
+			 * R_ARM_PREL31, mark unwind section .ARM.extab and .ARM.exidx as
+			 * executable so they will be allocated within .text section to meet
+			 * +/-1GB range requirement of the R_ARM_PREL31 relocation
+			 */
+			s->sh_flags |= SHF_EXECINSTR;
+		}
+#endif
 	}
 
 	if (!mod->arch.core.plt || !mod->arch.init.plt) {
-- 
2.43.7

Re: [PATCH] ARM: module: fix unwind section relocation out of range error

Posted by Kursad Oney 4 months, 2 weeks ago

Hi William,


On Fri, Sep 19, 2025 at 10:10 PM William Zhang
<william.zhang@broadcom.com> wrote:
>
> In an armv7 system that uses non-3G/1G split and with more than 512MB
> physical memory, driver load may fail with following error:
>    section 29 reloc 0 sym '': relocation 42 out of range (0xc2ab9be8 ->
> 0x7fad5998)
>
> This happens when relocation R_ARM_PREL31 from the unwind section
> .ARM.extab and .ARM.exidx are allocated from the VMALLOC space while
> .text section is from MODULES_VADDR space. It exceeds the +/-1GB
> relocation requirement of R_ARM_PREL31 hence triggers the error.
>
> The fix is to mark .ARM.extab and .ARM.exidx sections as executable so
> they can be allocated within .text section and always meet range
> requirement.

Not "within" .text, but "along with" or "in close proximity to".

>
> Co-developed-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: William Zhang <william.zhang@broadcom.com>

I think a Fixes tag might be appropriate?

>
> ---
>
>  arch/arm/kernel/module-plts.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/arch/arm/kernel/module-plts.c b/arch/arm/kernel/module-plts.c
> index 354ce16d83cb..5f5bf5e63bd6 100644
> --- a/arch/arm/kernel/module-plts.c
> +++ b/arch/arm/kernel/module-plts.c
> @@ -225,6 +225,18 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
>                         mod->arch.init.plt = s;
>                 else if (s->sh_type == SHT_SYMTAB)
>                         syms = (Elf32_Sym *)s->sh_addr;
> +#if defined(CONFIG_ARM_UNWIND) && !defined(CONFIG_VMSPLIT_3G)
> +               else if (s->sh_type == ELF_SECTION_UNWIND ||
> +                        (strncmp(".ARM.extab", secstrings + s->sh_name, 10) == 0)) {
> +                       /*
> +                        * To avoid the possible relocation out of range issue for
> +                        * R_ARM_PREL31, mark unwind section .ARM.extab and .ARM.exidx as
> +                        * executable so they will be allocated within .text section to meet
> +                        * +/-1GB range requirement of the R_ARM_PREL31 relocation
> +                        */
> +                       s->sh_flags |= SHF_EXECINSTR;
> +               }
> +#endif
>         }
>
>         if (!mod->arch.core.plt || !mod->arch.init.plt) {
> --
> 2.43.7
>

Re: [PATCH] ARM: module: fix unwind section relocation out of range error

Posted by William Zhang 4 months, 2 weeks ago

Hi Kursad,

On Mon, Sep 22, 2025 at 6:13 AM Kursad Oney <kursad.oney@broadcom.com> wrote:
>
> Hi William,
>
>
> On Fri, Sep 19, 2025 at 10:10 PM William Zhang
> <william.zhang@broadcom.com> wrote:
> >
> > In an armv7 system that uses non-3G/1G split and with more than 512MB
> > physical memory, driver load may fail with following error:
> >    section 29 reloc 0 sym '': relocation 42 out of range (0xc2ab9be8 ->
> > 0x7fad5998)
> >
> > This happens when relocation R_ARM_PREL31 from the unwind section
> > .ARM.extab and .ARM.exidx are allocated from the VMALLOC space while
> > .text section is from MODULES_VADDR space. It exceeds the +/-1GB
> > relocation requirement of R_ARM_PREL31 hence triggers the error.
> >
> > The fix is to mark .ARM.extab and .ARM.exidx sections as executable so
> > they can be allocated within .text section and always meet range
> > requirement.
>
> Not "within" .text, but "along with" or "in close proximity to".
Yeah that is more accurate.  Will update.

>
> >
> > Co-developed-by: Ard Biesheuvel <ardb@kernel.org>
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > Signed-off-by: William Zhang <william.zhang@broadcom.com>
>
> I think a Fixes tag might be appropriate?
Yes, the change ac3b43283923 ("module: replace module_layout with
module_memory") does expose this issue. But I won't say that change
itself has a bug and it is more of an enhancement that this arm module
code has to handle different memory allocation scenarios.
>
> >
> > ---
> >
> >  arch/arm/kernel/module-plts.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/arch/arm/kernel/module-plts.c b/arch/arm/kernel/module-plts.c
> > index 354ce16d83cb..5f5bf5e63bd6 100644
> > --- a/arch/arm/kernel/module-plts.c
> > +++ b/arch/arm/kernel/module-plts.c
> > @@ -225,6 +225,18 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
> >                         mod->arch.init.plt = s;
> >                 else if (s->sh_type == SHT_SYMTAB)
> >                         syms = (Elf32_Sym *)s->sh_addr;
> > +#if defined(CONFIG_ARM_UNWIND) && !defined(CONFIG_VMSPLIT_3G)
> > +               else if (s->sh_type == ELF_SECTION_UNWIND ||
> > +                        (strncmp(".ARM.extab", secstrings + s->sh_name, 10) == 0)) {
> > +                       /*
> > +                        * To avoid the possible relocation out of range issue for
> > +                        * R_ARM_PREL31, mark unwind section .ARM.extab and .ARM.exidx as
> > +                        * executable so they will be allocated within .text section to meet
> > +                        * +/-1GB range requirement of the R_ARM_PREL31 relocation
> > +                        */
> > +                       s->sh_flags |= SHF_EXECINSTR;
> > +               }
> > +#endif
> >         }
> >
> >         if (!mod->arch.core.plt || !mod->arch.init.plt) {
> > --
> > 2.43.7
> >