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Subject[PATCH v2] arm64/module: use plt section indices for relocations
Instead of saving a pointer to the .plt and .init.plt sections to apply
plt-based relocations, save and use their section indices instead.

The mod->arch.{core,init}.plt pointers were problematic for livepatch
because they pointed within temporary section headers (provided by the
module loader via info->sechdrs) that would be freed after module load.
Since livepatch modules may need to apply relocations post-module-load
(for example, to patch a module that is loaded later), using section
indices to offset into the section headers (instead of accessing them
through a saved pointer) allows livepatch modules on arm64 to pass in
their own copy of the section headers to apply_relocate_add() to apply
delayed relocations.

Signed-off-by: Jessica Yu <jeyu@kernel.org>
---

v2:

- Do sechdrs[pltsec->plt_shndx].sh_addr instead of pointer math

Note: Addressed Will's comment about the pltsec -> plt_info rename and
removed that change to reduce unnecessary code churn. I didn't include the
Ack's for this reason so let me know if this version is OK as well.

arch/arm64/include/asm/module.h | 8 +++++---
arch/arm64/kernel/module-plts.c | 36 ++++++++++++++++++++----------------
arch/arm64/kernel/module.c | 9 +++++----
3 files changed, 30 insertions(+), 23 deletions(-)

diff --git a/arch/arm64/include/asm/module.h b/arch/arm64/include/asm/module.h
index 97d0ef12e2ff..f81c1847312d 100644
--- a/arch/arm64/include/asm/module.h
+++ b/arch/arm64/include/asm/module.h
@@ -22,7 +22,7 @@

#ifdef CONFIG_ARM64_MODULE_PLTS
struct mod_plt_sec {
- struct elf64_shdr *plt;
+ int plt_shndx;
int plt_num_entries;
int plt_max_entries;
};
@@ -36,10 +36,12 @@ struct mod_arch_specific {
};
#endif

-u64 module_emit_plt_entry(struct module *mod, void *loc, const Elf64_Rela *rela,
+u64 module_emit_plt_entry(struct module *mod, Elf64_Shdr *sechdrs,
+ void *loc, const Elf64_Rela *rela,
Elf64_Sym *sym);

-u64 module_emit_veneer_for_adrp(struct module *mod, void *loc, u64 val);
+u64 module_emit_veneer_for_adrp(struct module *mod, Elf64_Shdr *sechdrs,
+ void *loc, u64 val);

#ifdef CONFIG_RANDOMIZE_BASE
extern u64 module_alloc_base;
diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
index f0690c2ca3e0..a0efe30211d6 100644
--- a/arch/arm64/kernel/module-plts.c
+++ b/arch/arm64/kernel/module-plts.c
@@ -16,12 +16,13 @@ static bool in_init(const struct module *mod, void *loc)
return (u64)loc - (u64)mod->init_layout.base < mod->init_layout.size;
}

-u64 module_emit_plt_entry(struct module *mod, void *loc, const Elf64_Rela *rela,
+u64 module_emit_plt_entry(struct module *mod, Elf64_Shdr *sechdrs,
+ void *loc, const Elf64_Rela *rela,
Elf64_Sym *sym)
{
struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core :
&mod->arch.init;
- struct plt_entry *plt = (struct plt_entry *)pltsec->plt->sh_addr;
+ struct plt_entry *plt = (struct plt_entry *)sechdrs[pltsec->plt_shndx].sh_addr;
int i = pltsec->plt_num_entries;
u64 val = sym->st_value + rela->r_addend;

@@ -43,11 +44,12 @@ u64 module_emit_plt_entry(struct module *mod, void *loc, const Elf64_Rela *rela,
}

#ifdef CONFIG_ARM64_ERRATUM_843419
-u64 module_emit_veneer_for_adrp(struct module *mod, void *loc, u64 val)
+u64 module_emit_veneer_for_adrp(struct module *mod, Elf64_Shdr *sechdrs,
+ void *loc, u64 val)
{
struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core :
&mod->arch.init;
- struct plt_entry *plt = (struct plt_entry *)pltsec->plt->sh_addr;
+ struct plt_entry *plt = (struct plt_entry *)sechdrs[pltsec->plt_shndx].sh_addr;
int i = pltsec->plt_num_entries++;
u32 mov0, mov1, mov2, br;
int rd;
@@ -202,7 +204,7 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
unsigned long core_plts = 0;
unsigned long init_plts = 0;
Elf64_Sym *syms = NULL;
- Elf_Shdr *tramp = NULL;
+ Elf_Shdr *pltsec, *tramp = NULL;
int i;

/*
@@ -211,9 +213,9 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
*/
for (i = 0; i < ehdr->e_shnum; i++) {
if (!strcmp(secstrings + sechdrs[i].sh_name, ".plt"))
- mod->arch.core.plt = sechdrs + i;
+ mod->arch.core.plt_shndx = i;
else if (!strcmp(secstrings + sechdrs[i].sh_name, ".init.plt"))
- mod->arch.init.plt = sechdrs + i;
+ mod->arch.init.plt_shndx = i;
else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
!strcmp(secstrings + sechdrs[i].sh_name,
".text.ftrace_trampoline"))
@@ -222,7 +224,7 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
syms = (Elf64_Sym *)sechdrs[i].sh_addr;
}

- if (!mod->arch.core.plt || !mod->arch.init.plt) {
+ if (!mod->arch.core.plt_shndx || !mod->arch.init.plt_shndx) {
pr_err("%s: module PLT section(s) missing\n", mod->name);
return -ENOEXEC;
}
@@ -254,17 +256,19 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
sechdrs[i].sh_info, dstsec);
}

- mod->arch.core.plt->sh_type = SHT_NOBITS;
- mod->arch.core.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
- mod->arch.core.plt->sh_addralign = L1_CACHE_BYTES;
- mod->arch.core.plt->sh_size = (core_plts + 1) * sizeof(struct plt_entry);
+ pltsec = sechdrs + mod->arch.core.plt_shndx;
+ pltsec->sh_type = SHT_NOBITS;
+ pltsec->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
+ pltsec->sh_addralign = L1_CACHE_BYTES;
+ pltsec->sh_size = (core_plts + 1) * sizeof(struct plt_entry);
mod->arch.core.plt_num_entries = 0;
mod->arch.core.plt_max_entries = core_plts;

- mod->arch.init.plt->sh_type = SHT_NOBITS;
- mod->arch.init.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
- mod->arch.init.plt->sh_addralign = L1_CACHE_BYTES;
- mod->arch.init.plt->sh_size = (init_plts + 1) * sizeof(struct plt_entry);
+ pltsec = sechdrs + mod->arch.init.plt_shndx;
+ pltsec->sh_type = SHT_NOBITS;
+ pltsec->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
+ pltsec->sh_addralign = L1_CACHE_BYTES;
+ pltsec->sh_size = (init_plts + 1) * sizeof(struct plt_entry);
mod->arch.init.plt_num_entries = 0;
mod->arch.init.plt_max_entries = init_plts;

diff --git a/arch/arm64/kernel/module.c b/arch/arm64/kernel/module.c
index f0f27aeefb73..c2abe59c6f8f 100644
--- a/arch/arm64/kernel/module.c
+++ b/arch/arm64/kernel/module.c
@@ -198,7 +198,8 @@ static int reloc_insn_imm(enum aarch64_reloc_op op, __le32 *place, u64 val,
return 0;
}

-static int reloc_insn_adrp(struct module *mod, __le32 *place, u64 val)
+static int reloc_insn_adrp(struct module *mod, Elf64_Shdr *sechdrs,
+ __le32 *place, u64 val)
{
u32 insn;

@@ -215,7 +216,7 @@ static int reloc_insn_adrp(struct module *mod, __le32 *place, u64 val)
insn &= ~BIT(31);
} else {
/* out of range for ADR -> emit a veneer */
- val = module_emit_veneer_for_adrp(mod, place, val & ~0xfff);
+ val = module_emit_veneer_for_adrp(mod, sechdrs, place, val & ~0xfff);
if (!val)
return -ENOEXEC;
insn = aarch64_insn_gen_branch_imm((u64)place, val,
@@ -368,7 +369,7 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
case R_AARCH64_ADR_PREL_PG_HI21_NC:
overflow_check = false;
case R_AARCH64_ADR_PREL_PG_HI21:
- ovf = reloc_insn_adrp(me, loc, val);
+ ovf = reloc_insn_adrp(me, sechdrs, loc, val);
if (ovf && ovf != -ERANGE)
return ovf;
break;
@@ -413,7 +414,7 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,

if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
ovf == -ERANGE) {
- val = module_emit_plt_entry(me, loc, &rel[i], sym);
+ val = module_emit_plt_entry(me, sechdrs, loc, &rel[i], sym);
if (!val)
return -ENOEXEC;
ovf = reloc_insn_imm(RELOC_OP_PREL, loc, val, 2,
--
2.14.5
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