Re: [PATCH v2 4/8] x86: add support for 64-bit place relative relocations
Hi Ard,
I love your patch! Yet something to improve:
[auto build test ERROR on linus/master]
[also build test ERROR on v4.18-rc3 next-20180702]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system]
url:
https://github.com/0day-ci/linux/commits/Ard-Biesheuvel/add-support-for-relative-references-in-jump-tables/20180703-031712
config: um-x86_64_defconfig (attached as .config)
compiler: gcc-7 (Debian 7.3.0-16) 7.3.0
reproduce:
# save the attached .config to linux build tree
make ARCH=um SUBARCH=x86_64
All errors (new ones prefixed by >>):
arch/x86/um/../kernel/module.c: In function 'apply_relocate_add':
>> arch/x86/um/../kernel/module.c:204:8: error: 'R_X86_64_PC64' undeclared
>> (first use in this function); did you mean 'R_X86_64_PC16'?
case R_X86_64_PC64:
^
R_X86_64_PC16
arch/x86/um/../kernel/module.c:204:8: note: each undeclared identifier is
reported only once for each function it appears in
vim +204 arch/x86/um/../kernel/module.c
99
100 #ifdef CONFIG_X86_32
101 int apply_relocate(Elf32_Shdr *sechdrs,
102 const char *strtab,
103 unsigned int symindex,
104 unsigned int relsec,
105 struct module *me)
106 {
107 unsigned int i;
108 Elf32_Rel *rel = (void *)sechdrs[relsec].sh_addr;
109 Elf32_Sym *sym;
110 uint32_t *location;
111
112 DEBUGP("Applying relocate section %u to %u\n",
113 relsec, sechdrs[relsec].sh_info);
114 for (i = 0; i < sechdrs[relsec].sh_size / sizeof(*rel); i++) {
115 /* This is where to make the change */
116 location = (void
*)sechdrs[sechdrs[relsec].sh_info].sh_addr
117 + rel[i].r_offset;
118 /* This is the symbol it is referring to. Note that all
119 undefined symbols have been resolved. */
120 sym = (Elf32_Sym *)sechdrs[symindex].sh_addr
121 + ELF32_R_SYM(rel[i].r_info);
122
123 switch (ELF32_R_TYPE(rel[i].r_info)) {
124 case R_386_32:
125 /* We add the value into the location given */
126 *location += sym->st_value;
127 break;
128 case R_386_PC32:
129 /* Add the value, subtract its position */
130 *location += sym->st_value - (uint32_t)location;
131 break;
132 default:
133 pr_err("%s: Unknown relocation: %u\n",
134 me->name, ELF32_R_TYPE(rel[i].r_info));
135 return -ENOEXEC;
136 }
137 }
138 return 0;
139 }
140 #else /*X86_64*/
141 int apply_relocate_add(Elf64_Shdr *sechdrs,
142 const char *strtab,
143 unsigned int symindex,
144 unsigned int relsec,
145 struct module *me)
146 {
147 unsigned int i;
148 Elf64_Rela *rel = (void *)sechdrs[relsec].sh_addr;
149 Elf64_Sym *sym;
150 void *loc;
151 u64 val;
152
153 DEBUGP("Applying relocate section %u to %u\n",
154 relsec, sechdrs[relsec].sh_info);
155 for (i = 0; i < sechdrs[relsec].sh_size / sizeof(*rel); i++) {
156 /* This is where to make the change */
157 loc = (void *)sechdrs[sechdrs[relsec].sh_info].sh_addr
158 + rel[i].r_offset;
159
160 /* This is the symbol it is referring to. Note that all
161 undefined symbols have been resolved. */
162 sym = (Elf64_Sym *)sechdrs[symindex].sh_addr
163 + ELF64_R_SYM(rel[i].r_info);
164
165 DEBUGP("type %d st_value %Lx r_addend %Lx loc %Lx\n",
166 (int)ELF64_R_TYPE(rel[i].r_info),
167 sym->st_value, rel[i].r_addend, (u64)loc);
168
169 val = sym->st_value + rel[i].r_addend;
170
171 switch (ELF64_R_TYPE(rel[i].r_info)) {
172 case R_X86_64_NONE:
173 break;
174 case R_X86_64_64:
175 if (*(u64 *)loc != 0)
176 goto invalid_relocation;
177 *(u64 *)loc = val;
178 break;
179 case R_X86_64_32:
180 if (*(u32 *)loc != 0)
[PATCH v2 4/8] x86: add support for 64-bit place relative relocations
Add support for R_X86_64_PC64 relocations, which operate on 64-bit
quantities holding a relative symbol reference. This allows jump
table entries to be emitted in a way that makes them invariant under
runtime relocation, which means that no metadata needs to be emitted
into the kernel image to describe such data structures, resulting in
a size reduction.
Signed-off-by: Ard Biesheuvel
---
arch/x86/include/asm/elf.h | 1 +
arch/x86/kernel/machine_kexec_64.c | 4
arch/x86/kernel/module.c | 6 ++
arch/x86/tools/relocs.c| 10 ++
4 files changed, 21 insertions(+)
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 0d157d2a1e2a..d3925d684296 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -62,6 +62,7 @@ typedef struct user_fxsr_struct elf_fpxregset_t;
#define R_X86_64_PC16 13 /* 16 bit sign extended pc relative */
#define R_X86_64_8 14 /* Direct 8 bit sign extended */
#define R_X86_64_PC8 15 /* 8 bit sign extended pc relative */
+#define R_X86_64_PC64 24 /* Place relative 64-bit signed */
#define R_X86_64_NUM 16
diff --git a/arch/x86/kernel/machine_kexec_64.c
b/arch/x86/kernel/machine_kexec_64.c
index 4c8acdfdc5a7..6638d1edb2be 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -496,6 +496,10 @@ int arch_kexec_apply_relocations_add(struct purgatory_info
*pi,
value -= (u64)address;
*(u32 *)location = value;
break;
+ case R_X86_64_PC64:
+ value -= (u64)address;
+ *(u64 *)location = value;
+ break;
default:
pr_err("Unknown rela relocation: %llu\n",
ELF64_R_TYPE(rel[i].r_info));
diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index f58336af095c..b052e883dd8c 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -201,6 +201,12 @@ int apply_relocate_add(Elf64_Shdr *sechdrs,
goto overflow;
#endif
break;
+ case R_X86_64_PC64:
+ if (*(u64 *)loc != 0)
+ goto invalid_relocation;
+ val -= (u64)loc;
+ *(u64 *)loc = val;
+ break;
default:
pr_err("%s: Unknown rela relocation: %llu\n",
me->name, ELF64_R_TYPE(rel[i].r_info));
diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index 220e97841e49..a4075bc37e8f 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -195,6 +195,7 @@ static const char *rel_type(unsigned type)
#if ELF_BITS == 64
REL_TYPE(R_X86_64_NONE),
REL_TYPE(R_X86_64_64),
+ REL_TYPE(R_X86_64_PC64),
REL_TYPE(R_X86_64_PC32),
REL_TYPE(R_X86_64_GOT32),
REL_TYPE(R_X86_64_PLT32),
@@ -781,6 +782,15 @@ static int do_reloc64(struct section *sec, Elf_Rel *rel,
ElfW(Sym) *sym,
add_reloc(&relocs32neg, offset);
break;
+ case R_X86_64_PC64:
+ /*
+* Only used by jump labels
+*/
+ if (is_percpu_sym(sym, symname))
+ die("Invalid R_X86_64_PC64 relocation against per-CPU
symbol %s\n",
+ symname);
+ break;
+
case R_X86_64_32:
case R_X86_64_32S:
case R_X86_64_64:
--
2.17.1
