Re: [PATCH 04/19] ARM: implement elf_apply_relocations() for ELF relocation support

[Sascha Hauer <s.hauer@pengutronix.de>]

On Mon, Jan 05, 2026 at 12:58:23PM +0100, Ahmad Fatoum wrote:
> Hi,
> 
> On 1/5/26 12:26 PM, Sascha Hauer wrote:
> > Implement architecture-specific ELF relocation handlers for ARM32 and ARM64.
> > 
> > ARM32 implementation (arch/arm/lib32/elf_reloc.c):
> > - Handles REL-format relocations (no explicit addend)
> > - Supports R_ARM_RELATIVE and R_ARM_ABS32 relocation types
> > - Addend is read from the target location
> > 
> > ARM64 implementation (arch/arm/lib64/elf_reloc.c):
> > - Handles RELA-format relocations (with explicit addend)
> > - Supports R_AARCH64_RELATIVE and R_AARCH64_ABS64 relocation types
> > - Addend is provided in relocation entry
> > 
> > Both implementations:
> > - Parse PT_DYNAMIC segment to locate relocation tables
> > - Validate relocation table format and entry sizes
> > - Apply relocations based on the computed load offset
> > - Return appropriate errors for unsupported relocation types
> > 
> > Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
> > Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
> > ---
> >  arch/arm/include/asm/elf.h |  11 +++++
> >  arch/arm/lib32/Makefile    |   1 +
> >  arch/arm/lib32/elf_reloc.c | 105 +++++++++++++++++++++++++++++++++++++++++++++
> >  arch/arm/lib64/Makefile    |   1 +
> >  arch/arm/lib64/elf_reloc.c | 105 +++++++++++++++++++++++++++++++++++++++++++++
> >  5 files changed, 223 insertions(+)
> > 
> > diff --git a/arch/arm/include/asm/elf.h b/arch/arm/include/asm/elf.h
> > index 4043e6fd5b991eb5cccb3fa0ea28d208006ee1fc..cceb92ee1a5f63c37b0e981c263676bd35a261c0 100644
> > --- a/arch/arm/include/asm/elf.h
> > +++ b/arch/arm/include/asm/elf.h
> > @@ -36,6 +36,17 @@ typedef struct user_fp elf_fpregset_t;
> >  #define R_ARM_THM_CALL		10
> >  #define R_ARM_THM_JUMP24	30
> >  
> > +/* Additional relocation types for dynamic linking */
> > +#define R_ARM_RELATIVE		23
> 
> This is already defined in arch/arm/cpu/common.c, you can guess where
> this is going ;)
> 
> > +#define R_ARM_GLOB_DAT		21
> > +#define R_ARM_JUMP_SLOT		22
> 
> These two are not used.
> 
> > +
> > +#define R_AARCH64_NONE		0
> > +#define R_AARCH64_ABS64		257
> > +#define R_AARCH64_RELATIVE	1027
> > +#define R_AARCH64_GLOB_DAT	1025
> > +#define R_AARCH64_JUMP_SLOT	1026
> 
> Likewise, only R_AARCH64_JUMP_SLOT and R_AARCH64_GLOB_DAT are not used.
> 
> > +/*
> > + * Parse dynamic section and extract relocation info for ARM32
> > + */
> > +static int parse_dynamic_section(struct elf_image *elf, Elf32_Dyn *dyn,
> > +				  Elf32_Rel **rel_out, u64 *relsz_out)
> > +{
> > +	Elf32_Rel *rel = NULL;
> 
> If we define ELF_CLASS depending on CONFIG_ARM32/CONFIG_ARM64, we can
> write a generic parse_dynamic_section() that applies to all architectures.
> 
> In this case, we would just collect both REL and RELA in the switch and
> just use what's available.
> > +int elf_apply_relocations(struct elf_image *elf, void *dyn_seg)
> > +{
> 
> We already have relocate_to_current_adr(). If we change it to call
> a new relocate_image():
> 
> 	relocate_image(get_runtime_offset(),
> 			runtime_addr(__rel_dyn_start),
> 			runtime_addr(__rel_dyn_end),
> 			runtime_addr(__dynsym_start),
> 			runtime_addr(__dynsym_end));
> 
> Then we could use relocate_image() in generic code instead of
> duplicating it per architecture.
> 
> > +/*
> > + * Parse dynamic section and extract relocation info for ARM64
> > + */
> > +static int parse_dynamic_section(struct elf_image *elf, Elf64_Dyn *dyn,
> > +				  Elf64_Rela **rela_out, u64 *relasz_out)
> 
> Same comment as for ARM32. I believe this can go into generic code.
> 
> > +		case R_AARCH64_ABS64:
> > +			/* B(P) = S + A */
> > +			*fixup_addr = base + rela->r_addend;
> > +			break;
> 
> What did you do to get R_AARCH64_ABS64 relocations? We don't handle them
> in relocate_to_current_adr() and they shouldn't have been generated.

Likely Claude just generated this for completeness.

Sascha

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