[PATCH v3 17/27] x86/relocs: Handle PIE relocations

Thomas Garnier
2018-05-23T19:54:11Z

Change the relocation tool to correctly handle relocations generated by
-fPIE option:

- Add relocation for each entry of the .got section given the linker does not
generate R_X86_64_GLOB_DAT on a simple link.
- Ignore R_X86_64_GOTPCREL.

Signed-off-by: Thomas Garnier <thgarnie@google.com>
---
arch/x86/tools/relocs.c | 93 ++++++++++++++++++++++++++++++++++++++++-
1 file changed, 92 insertions(+), 1 deletion(-)

diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c
index 220e97841e49..a35cc337f883 100644
--- a/arch/x86/tools/relocs.c
+++ b/arch/x86/tools/relocs.c
@@ -32,6 +32,7 @@ struct section {
Elf_Sym *symtab;
Elf_Rel *reltab;
char *strtab;
+ Elf_Addr *got;
};
static struct section *secs;

@@ -293,6 +294,35 @@ static Elf_Sym *sym_lookup(const char *symname)
return 0;
}

+static Elf_Sym *sym_lookup_addr(Elf_Addr addr, const char **name)
+{
+ int i;
+ for (i = 0; i < ehdr.e_shnum; i++) {
+ struct section *sec = &secs[i];
+ long nsyms;
+ Elf_Sym *symtab;
+ Elf_Sym *sym;
+
+ if (sec->shdr.sh_type != SHT_SYMTAB)
+ continue;
+
+ nsyms = sec->shdr.sh_size/sizeof(Elf_Sym);
+ symtab = sec->symtab;
+
+ for (sym = symtab; --nsyms >= 0; sym++) {
+ if (sym->st_value == addr) {
+ if (name) {
+ *name = sym_name(sec->link->strtab,
+ sym);
+ }
+ return sym;
+ }
+ }
+ }
+ return 0;
+}
+
+
#if BYTE_ORDER == LITTLE_ENDIAN
#define le16_to_cpu(val) (val)
#define le32_to_cpu(val) (val)
@@ -513,6 +543,33 @@ static void read_relocs(FILE *fp)
}
}

+static void read_got(FILE *fp)
+{
+ int i;
+ for (i = 0; i < ehdr.e_shnum; i++) {
+ struct section *sec = &secs[i];
+ sec->got = NULL;
+ if (sec->shdr.sh_type != SHT_PROGBITS ||
+ strcmp(sec_name(i), ".got")) {
+ continue;
+ }
+ sec->got = malloc(sec->shdr.sh_size);
+ if (!sec->got) {
+ die("malloc of %d bytes for got failed\n",
+ sec->shdr.sh_size);
+ }
+ if (fseek(fp, sec->shdr.sh_offset, SEEK_SET) < 0) {
+ die("Seek to %d failed: %s\n",
+ sec->shdr.sh_offset, strerror(errno));
+ }
+ if (fread(sec->got, 1, sec->shdr.sh_size, fp)
+ != sec->shdr.sh_size) {
+ die("Cannot read got: %s\n",
+ strerror(errno));
+ }
+ }
+}
+

static void print_absolute_symbols(void)
{
@@ -643,6 +700,32 @@ static void add_reloc(struct relocs *r, uint32_t offset)
r->offset[r->count++] = offset;
}

+/*
+ * The linker does not generate relocations for the GOT for the kernel.
+ * If a GOT is found, simulate the relocations that should have been included.
+ */
+static void walk_got_table(int (*process)(struct section *sec, Elf_Rel *rel,
+ Elf_Sym *sym, const char *symname),
+ struct section *sec)
+{
+ int i;
+ Elf_Addr entry;
+ Elf_Sym *sym;
+ const char *symname;
+ Elf_Rel rel;
+
+ for (i = 0; i < sec->shdr.sh_size/sizeof(Elf_Addr); i++) {
+ entry = sec->got[i];
+ sym = sym_lookup_addr(entry, &symname);
+ if (!sym)
+ die("Could not found got symbol for entry %d\n", i);
+ rel.r_offset = sec->shdr.sh_addr + i * sizeof(Elf_Addr);
+ rel.r_info = ELF_BITS == 64 ? R_X86_64_GLOB_DAT
+ : R_386_GLOB_DAT;
+ process(sec, &rel, sym, symname);
+ }
+}
+
static void walk_relocs(int (*process)(struct section *sec, Elf_Rel *rel,
Elf_Sym *sym, const char *symname))
{
@@ -656,6 +739,8 @@ static void walk_relocs(int (*process)(struct section *sec, Elf_Rel *rel,
struct section *sec = &secs[i];

if (sec->shdr.sh_type != SHT_REL_TYPE) {
+ if (sec->got)
+ walk_got_table(process, sec);
continue;
}
sec_symtab = sec->link;
@@ -765,6 +850,7 @@ static int do_reloc64(struct section *sec, Elf_Rel *rel, ElfW(Sym) *sym,
offset += per_cpu_load_addr;

switch (r_type) {
+ case R_X86_64_GOTPCREL:
case R_X86_64_NONE:
/* NONE can be ignored. */
break;
@@ -809,7 +895,7 @@ static int do_reloc64(struct section *sec, Elf_Rel *rel, ElfW(Sym) *sym,
* the relocations are processed.
* Make sure that the offset will fit.
*/
- if ((int32_t)offset != (int64_t)offset)
+ if (r_type != R_X86_64_64 && (int32_t)offset != (int64_t)offset)
die("Relocation offset doesn't fit in 32 bits\n");

if (r_type == R_X86_64_64)
@@ -818,6 +904,10 @@ static int do_reloc64(struct section *sec, Elf_Rel *rel, ElfW(Sym) *sym,
add_reloc(&relocs32, offset);
break;

+ case R_X86_64_GLOB_DAT:
+ add_reloc(&relocs64, offset);
+ break;
+
default:
die("Unsupported relocation type: %s (%d)\n",
rel_type(r_type), r_type);
@@ -1087,6 +1177,7 @@ void process(FILE *fp, int use_real_mode, int as_text,
read_strtabs(fp);
read_symtabs(fp);
read_relocs(fp);
+ read_got(fp);
if (ELF_BITS == 64)
percpu_init();
if (show_absolute_syms) {