From cd1824fb7a377882497e8b87a6f3a9ec19be3623 Mon Sep 17 00:00:00 2001 From: Masahiro Yamada Date: Thu, 1 Jun 2023 21:09:59 +0900 Subject: [PATCH] modpost: detect section mismatch for R_ARM_THM_{MOVW_ABS_NC,MOVT_ABS} When CONFIG_THUMB2_KERNEL is enabled, modpost fails to detect some types of section mismatches. [test code] #include int __initdata foo; int get_foo(void) { return foo; } It is apparently a bad reference, but modpost does not report anything. The test code above produces the following relocations. Relocation section '.rel.text' at offset 0x1e8 contains 2 entries: Offset Info Type Sym.Value Sym. Name 00000000 0000052f R_ARM_THM_MOVW_AB 00000000 .LANCHOR0 00000004 00000530 R_ARM_THM_MOVT_AB 00000000 .LANCHOR0 Currently, R_ARM_THM_MOVW_ABS_NC and R_ARM_THM_MOVT_ABS are just skipped. Add code to handle them. I checked arch/arm/kernel/module.c to learn how the offset is encoded in the instruction. One more thing to note for Thumb instructions - the st_value is an odd value, so you need to mask the bit 0 to get the offset. Otherwise, you will get an off-by-one error in the nearest symbol look-up. It is documented in "ELF for the ARM Architecture" [1]: In addition to the normal rules for symbol values the following rules shall also apply to symbols of type STT_FUNC: * If the symbol addresses an Arm instruction, its value is the address of the instruction (in a relocatable object, the offset of the instruction from the start of the section containing it). * If the symbol addresses a Thumb instruction, its value is the address of the instruction with bit zero set (in a relocatable object, the section offset with bit zero set). * For the purposes of relocation the value used shall be the address of the instruction (st_value & ~1). [1]: https://github.com/ARM-software/abi-aa/blob/main/aaelf32/aaelf32.rst Signed-off-by: Masahiro Yamada --- scripts/mod/modpost.c | 31 ++++++++++++++++++++++++++----- 1 file changed, 26 insertions(+), 5 deletions(-) diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c index 32d56efe3f3b..ec16f4d7e55a 100644 --- a/scripts/mod/modpost.c +++ b/scripts/mod/modpost.c @@ -1082,7 +1082,8 @@ static Elf_Sym *find_nearest_sym(struct elf_info *elf, Elf_Addr addr, { Elf_Sym *sym; Elf_Sym *near = NULL; - Elf_Addr distance; + Elf_Addr sym_addr, distance; + bool is_arm = (elf->hdr->e_machine == EM_ARM); for (sym = elf->symtab_start; sym < elf->symtab_stop; sym++) { if (get_secindex(elf, sym) != secndx) @@ -1090,10 +1091,19 @@ static Elf_Sym *find_nearest_sym(struct elf_info *elf, Elf_Addr addr, if (!is_valid_name(elf, sym)) continue; - if (addr >= sym->st_value) - distance = addr - sym->st_value; + sym_addr = sym->st_value; + + /* + * For ARM Thumb instruction, the bit 0 of st_value is set + * if the symbol is STT_FUNC type. Mask it to get the address. + */ + if (is_arm && ELF_ST_TYPE(sym->st_info) == STT_FUNC) + sym_addr &= ~1; + + if (addr >= sym_addr) + distance = addr - sym_addr; else if (allow_negative) - distance = sym->st_value - addr; + distance = sym_addr - addr; else continue; @@ -1266,7 +1276,7 @@ static int addend_arm_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r) unsigned int r_typ = ELF_R_TYPE(r->r_info); Elf_Sym *sym = elf->symtab_start + ELF_R_SYM(r->r_info); void *loc = reloc_location(elf, sechdr, r); - uint32_t inst; + uint32_t inst, upper, lower; int32_t offset; switch (r_typ) { @@ -1288,6 +1298,17 @@ static int addend_arm_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r) offset = sign_extend32((inst & 0x00ffffff) << 2, 25); r->r_addend = offset + sym->st_value + 8; break; + case R_ARM_THM_MOVW_ABS_NC: + case R_ARM_THM_MOVT_ABS: + upper = TO_NATIVE(*(uint16_t *)loc); + lower = TO_NATIVE(*((uint16_t *)loc + 1)); + offset = sign_extend32(((upper & 0x000f) << 12) | + ((upper & 0x0400) << 1) | + ((lower & 0x7000) >> 4) | + (lower & 0x00ff), + 15); + r->r_addend = offset + sym->st_value; + break; case R_ARM_THM_CALL: case R_ARM_THM_JUMP24: case R_ARM_THM_JUMP19: -- 2.20.1