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Date: Wed, 13 Oct 2021 14:22:18 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org, jpoimboe@...hat.com, andrew.cooper3@...rix.com
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org,
alexei.starovoitov@...il.com, ndesaulniers@...gle.com
Subject: [PATCH 1/9] objtool,x86: Replace alternatives with .retpoline_sites
Instead of writing complete alternatives, simply provide a list of all
the retpoline thunk calls. Then the kernel is free to do with them as
it pleases. Simpler code all-round.
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
arch/x86/kernel/vmlinux.lds.S | 12 +++
arch/x86/lib/retpoline.S | 42 ------------
tools/objtool/arch/x86/decode.c | 126 +++++++-----------------------------
tools/objtool/elf.c | 84 ------------------------
tools/objtool/include/objtool/elf.h | 1
5 files changed, 38 insertions(+), 227 deletions(-)
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -273,6 +273,18 @@ SECTIONS
}
/*
+ * List of instructions that call/jmp/jcc to retpoline thunks
+ * __x86_indirect_thunk_*(). These instructions can be patched along
+ * with alternatives, after which the section can be freed.
+ */
+ . = ALIGN(8);
+ .retpoline_sites : AT(ADDR(.retpoline_sites) - LOAD_OFFSET) {
+ __retpoline_sites = .;
+ *(.retpoline_sites)
+ __retpoline_sites_end = .;
+ }
+
+ /*
* struct alt_inst entries. From the header (alternative.h):
* "Alternative instructions for different CPU types or capabilities"
* Think locking instructions on spinlocks.
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -711,121 +711,47 @@ const char *arch_ret_insn(int len)
return ret[len-1];
}
-/* asm/alternative.h ? */
-
-#define ALTINSTR_FLAG_INV (1 << 15)
-#define ALT_NOT(feat) ((feat) | ALTINSTR_FLAG_INV)
-
-struct alt_instr {
- s32 instr_offset; /* original instruction */
- s32 repl_offset; /* offset to replacement instruction */
- u16 cpuid; /* cpuid bit set for replacement */
- u8 instrlen; /* length of original instruction */
- u8 replacementlen; /* length of new instruction */
-} __packed;
-
-static int elf_add_alternative(struct elf *elf,
- struct instruction *orig, struct symbol *sym,
- int cpuid, u8 orig_len, u8 repl_len)
+int arch_rewrite_retpolines(struct objtool_file *file)
{
- const int size = sizeof(struct alt_instr);
- struct alt_instr *alt;
+ struct instruction *insn;
struct section *sec;
- Elf_Scn *s;
-
- sec = find_section_by_name(elf, ".altinstructions");
- if (!sec) {
- sec = elf_create_section(elf, ".altinstructions",
- SHF_ALLOC, 0, 0);
-
- if (!sec) {
- WARN_ELF("elf_create_section");
- return -1;
- }
- }
-
- s = elf_getscn(elf->elf, sec->idx);
- if (!s) {
- WARN_ELF("elf_getscn");
- return -1;
- }
+ int idx;
- sec->data = elf_newdata(s);
- if (!sec->data) {
- WARN_ELF("elf_newdata");
- return -1;
+ sec = find_section_by_name(file->elf, ".retpoline_sites");
+ if (sec) {
+ WARN("file already has .retpoline_sites, skipping");
+ return 0;
}
- sec->data->d_size = size;
- sec->data->d_align = 1;
-
- alt = sec->data->d_buf = malloc(size);
- if (!sec->data->d_buf) {
- perror("malloc");
- return -1;
- }
- memset(sec->data->d_buf, 0, size);
+ idx = 0;
+ list_for_each_entry(insn, &file->retpoline_call_list, call_node)
+ idx++;
- if (elf_add_reloc_to_insn(elf, sec, sec->sh.sh_size,
- R_X86_64_PC32, orig->sec, orig->offset)) {
- WARN("elf_create_reloc: alt_instr::instr_offset");
- return -1;
- }
+ if (!idx)
+ return 0;
- if (elf_add_reloc(elf, sec, sec->sh.sh_size + 4,
- R_X86_64_PC32, sym, 0)) {
- WARN("elf_create_reloc: alt_instr::repl_offset");
+ sec = elf_create_section(file->elf, ".retpoline_sites", 0,
+ sizeof(int), idx);
+ if (!sec) {
+ WARN("elf_create_section: .retpoline_sites");
return -1;
}
- alt->cpuid = bswap_if_needed(cpuid);
- alt->instrlen = orig_len;
- alt->replacementlen = repl_len;
-
- sec->sh.sh_size += size;
- sec->changed = true;
-
- return 0;
-}
-
-#define X86_FEATURE_RETPOLINE ( 7*32+12)
-
-int arch_rewrite_retpolines(struct objtool_file *file)
-{
- struct instruction *insn;
- struct reloc *reloc;
- struct symbol *sym;
- char name[32] = "";
-
+ idx = 0;
list_for_each_entry(insn, &file->retpoline_call_list, call_node) {
- if (insn->type != INSN_JUMP_DYNAMIC &&
- insn->type != INSN_CALL_DYNAMIC)
- continue;
-
- if (!strcmp(insn->sec->name, ".text.__x86.indirect_thunk"))
- continue;
-
- reloc = insn->reloc;
-
- sprintf(name, "__x86_indirect_alt_%s_%s",
- insn->type == INSN_JUMP_DYNAMIC ? "jmp" : "call",
- reloc->sym->name + 21);
-
- sym = find_symbol_by_name(file->elf, name);
- if (!sym) {
- sym = elf_create_undef_symbol(file->elf, name);
- if (!sym) {
- WARN("elf_create_undef_symbol");
- return -1;
- }
- }
+ int *site = (int *)sec->data->d_buf + idx;
+ *site = 0;
- if (elf_add_alternative(file->elf, insn, sym,
- ALT_NOT(X86_FEATURE_RETPOLINE), 5, 5)) {
- WARN("elf_add_alternative");
+ if (elf_add_reloc_to_insn(file->elf, sec,
+ idx * sizeof(int),
+ R_X86_64_PC32,
+ insn->sec, insn->offset)) {
+ WARN("elf_add_reloc_to_insn: .retpoline_sites");
return -1;
}
+
+ idx++;
}
return 0;
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -740,90 +740,6 @@ static int elf_add_string(struct elf *el
return len;
}
-struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
-{
- struct section *symtab, *symtab_shndx;
- struct symbol *sym;
- Elf_Data *data;
- Elf_Scn *s;
-
- sym = malloc(sizeof(*sym));
- if (!sym) {
- perror("malloc");
- return NULL;
- }
- memset(sym, 0, sizeof(*sym));
-
- sym->name = strdup(name);
-
- sym->sym.st_name = elf_add_string(elf, NULL, sym->name);
- if (sym->sym.st_name == -1)
- return NULL;
-
- sym->sym.st_info = GELF_ST_INFO(STB_GLOBAL, STT_NOTYPE);
- // st_other 0
- // st_shndx 0
- // st_value 0
- // st_size 0
-
- symtab = find_section_by_name(elf, ".symtab");
- if (!symtab) {
- WARN("can't find .symtab");
- return NULL;
- }
-
- s = elf_getscn(elf->elf, symtab->idx);
- if (!s) {
- WARN_ELF("elf_getscn");
- return NULL;
- }
-
- data = elf_newdata(s);
- if (!data) {
- WARN_ELF("elf_newdata");
- return NULL;
- }
-
- data->d_buf = &sym->sym;
- data->d_size = sizeof(sym->sym);
- data->d_align = 1;
- data->d_type = ELF_T_SYM;
-
- sym->idx = symtab->sh.sh_size / sizeof(sym->sym);
-
- symtab->sh.sh_size += data->d_size;
- symtab->changed = true;
-
- symtab_shndx = find_section_by_name(elf, ".symtab_shndx");
- if (symtab_shndx) {
- s = elf_getscn(elf->elf, symtab_shndx->idx);
- if (!s) {
- WARN_ELF("elf_getscn");
- return NULL;
- }
-
- data = elf_newdata(s);
- if (!data) {
- WARN_ELF("elf_newdata");
- return NULL;
- }
-
- data->d_buf = &sym->sym.st_size; /* conveniently 0 */
- data->d_size = sizeof(Elf32_Word);
- data->d_align = 4;
- data->d_type = ELF_T_WORD;
-
- symtab_shndx->sh.sh_size += 4;
- symtab_shndx->changed = true;
- }
-
- sym->sec = find_section_by_index(elf, 0);
-
- elf_add_symbol(elf, sym);
-
- return sym;
-}
-
struct section *elf_create_section(struct elf *elf, const char *name,
unsigned int sh_flags, size_t entsize, int nr)
{
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -141,7 +141,6 @@ int elf_write_insn(struct elf *elf, stru
unsigned long offset, unsigned int len,
const char *insn);
int elf_write_reloc(struct elf *elf, struct reloc *reloc);
-struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name);
int elf_write(struct elf *elf);
void elf_close(struct elf *elf);
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