[PATCH v3 08/14] ARM: module: implement support for PC-relative group relocations
Ard Biesheuvel
ardb at kernel.org
Fri Dec 3 02:08:57 PST 2021
Add support for the R_ARM_ALU_PC_Gn_NC and R_ARM_LDR_PC_G2 group
relocations so we can use them in modules. These will be used to load
the current task pointer from a global variable without having to rely
on a literal pool entry to carry the address of this variable, which
would be slightly less efficient.
Acked-by: Linus Walleij <linus.walleij at linaro.org>
Acked-by: Nicolas Pitre <nico at fluxnic.net>
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
arch/arm/include/asm/elf.h | 3 +
arch/arm/kernel/module.c | 63 ++++++++++++++++++++
2 files changed, 66 insertions(+)
diff --git a/arch/arm/include/asm/elf.h b/arch/arm/include/asm/elf.h
index b8102a6ddf16..d68101655b74 100644
--- a/arch/arm/include/asm/elf.h
+++ b/arch/arm/include/asm/elf.h
@@ -61,6 +61,9 @@ typedef struct user_fp elf_fpregset_t;
#define R_ARM_MOVT_ABS 44
#define R_ARM_MOVW_PREL_NC 45
#define R_ARM_MOVT_PREL 46
+#define R_ARM_ALU_PC_G0_NC 57
+#define R_ARM_ALU_PC_G1_NC 59
+#define R_ARM_LDR_PC_G2 63
#define R_ARM_THM_CALL 10
#define R_ARM_THM_JUMP24 30
diff --git a/arch/arm/kernel/module.c b/arch/arm/kernel/module.c
index beac45e89ba6..22bdce7a7f95 100644
--- a/arch/arm/kernel/module.c
+++ b/arch/arm/kernel/module.c
@@ -68,6 +68,20 @@ bool module_exit_section(const char *name)
strstarts(name, ".ARM.exidx.exit");
}
+static u32 get_group_rem(u32 group, u32 *offset)
+{
+ u32 val = *offset;
+ u32 shift;
+ do {
+ shift = val ? (31 - __fls(val)) & ~1 : 32;
+ *offset = val;
+ if (!val)
+ break;
+ val &= 0xffffff >> shift;
+ } while (group--);
+ return shift;
+}
+
int
apply_relocate(Elf32_Shdr *sechdrs, const char *strtab, unsigned int symindex,
unsigned int relindex, struct module *module)
@@ -82,6 +96,7 @@ apply_relocate(Elf32_Shdr *sechdrs, const char *strtab, unsigned int symindex,
unsigned long loc;
Elf32_Sym *sym;
const char *symname;
+ u32 shift, group = 1;
s32 offset;
u32 tmp;
#ifdef CONFIG_THUMB2_KERNEL
@@ -212,6 +227,54 @@ apply_relocate(Elf32_Shdr *sechdrs, const char *strtab, unsigned int symindex,
*(u32 *)loc = __opcode_to_mem_arm(tmp);
break;
+ case R_ARM_ALU_PC_G0_NC:
+ group = 0;
+ fallthrough;
+ case R_ARM_ALU_PC_G1_NC:
+ tmp = __mem_to_opcode_arm(*(u32 *)loc);
+ offset = ror32(tmp & 0xff, (tmp & 0xf00) >> 7);
+ if (tmp & BIT(22))
+ offset = -offset;
+ offset += sym->st_value - loc;
+ if (offset < 0) {
+ offset = -offset;
+ tmp = (tmp & ~BIT(23)) | BIT(22); // SUB opcode
+ } else {
+ tmp = (tmp & ~BIT(22)) | BIT(23); // ADD opcode
+ }
+
+ shift = get_group_rem(group, &offset);
+ if (shift < 24) {
+ offset >>= 24 - shift;
+ offset |= (shift + 8) << 7;
+ }
+ *(u32 *)loc = __opcode_to_mem_arm((tmp & ~0xfff) | offset);
+ break;
+
+ case R_ARM_LDR_PC_G2:
+ tmp = __mem_to_opcode_arm(*(u32 *)loc);
+ offset = tmp & 0xfff;
+ if (~tmp & BIT(23)) // U bit cleared?
+ offset = -offset;
+ offset += sym->st_value - loc;
+ if (offset < 0) {
+ offset = -offset;
+ tmp &= ~BIT(23); // clear U bit
+ } else {
+ tmp |= BIT(23); // set U bit
+ }
+ get_group_rem(2, &offset);
+
+ if (offset > 0xfff) {
+ pr_err("%s: section %u reloc %u sym '%s': relocation %u out of range (%#lx -> %#x)\n",
+ module->name, relindex, i, symname,
+ ELF32_R_TYPE(rel->r_info), loc,
+ sym->st_value);
+ return -ENOEXEC;
+ }
+ *(u32 *)loc = __opcode_to_mem_arm((tmp & ~0xfff) | offset);
+ break;
+
#ifdef CONFIG_THUMB2_KERNEL
case R_ARM_THM_CALL:
case R_ARM_THM_JUMP24:
--
2.30.2
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