pa-fazil
/
android_bionic
mirror of https://github.com/fazilsheik96/android_bionic.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

linker: find AT_BASE using AT_PHDR/AT_PHNUM 

When the linker is invoked directly, rather than as an interpreter for a
real program, the AT_BASE value is 0. To find the linker's base address,
the linker currently relies on the static linker populating the target of
a RELA relocation with an offset rather than leaving it zero. (With lld,
it will require a special flag, --apply-dynamic-relocs.)

Instead, do something more straightforward: the linker already finds the
executable's base address using its PHDR table, so do the same thing when
the linker is run by itself.

Bug: http://b/72789859
Test: boots, run linker/linker64 by itself
Change-Id: I4da5c346ca164ea6f4fbc011f8c3db4e6a829456
This commit is contained in:
Ryan Prichard 2018-07-13 22:40:26 -07:00
parent 3bf897e12d
commit 9729f35922
1 changed files with 32 additions and 33 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

65
linker/linker_main.cpp
View File

@ -56,6 +56,9 @@ extern "C" void _start();
static ElfW(Addr) get_elf_exec_load_bias(const ElfW(Ehdr)* elf);
static void get_elf_base_from_phdr(const ElfW(Phdr)* phdr_table, size_t phdr_count,
ElfW(Addr)* base, ElfW(Addr)* load_bias);
// These should be preserved static to avoid emitting
// RELATIVE relocations for the part of the code running
// before linker links itself.
@ -321,24 +324,8 @@ static ElfW(Addr) linker_main(KernelArgumentBlock& args) {
si->phdr = reinterpret_cast<ElfW(Phdr)*>(args.getauxval(AT_PHDR));
si->phnum = args.getauxval(AT_PHNUM);
/* Compute the value of si->base. We can't rely on the fact that
* the first entry is the PHDR because this will not be true
* for certain executables (e.g. some in the NDK unit test suite)
*/
si->base = 0;
get_elf_base_from_phdr(si->phdr, si->phnum, &si->base, &si->load_bias);
si->size = phdr_table_get_load_size(si->phdr, si->phnum);
si->load_bias = 0;
for (size_t i = 0; i < si->phnum; ++i) {
if (si->phdr[i].p_type == PT_PHDR) {
si->load_bias = reinterpret_cast<ElfW(Addr)>(si->phdr) - si->phdr[i].p_vaddr;
si->base = reinterpret_cast<ElfW(Addr)>(si->phdr) - si->phdr[i].p_offset;
break;
}
}
if (si->base == 0) {
async_safe_fatal("Could not find a PHDR: broken executable?");
}
si->dynamic = nullptr;
@ -503,6 +490,23 @@ static ElfW(Addr) get_elf_exec_load_bias(const ElfW(Ehdr)* elf) {
return 0;
}
/* Find the load bias and base address of an executable or shared object loaded
* by the kernel. The ELF file's PHDR table must have a PT_PHDR entry.
*
* A VDSO doesn't have a PT_PHDR entry in its PHDR table.
*/
static void get_elf_base_from_phdr(const ElfW(Phdr)* phdr_table, size_t phdr_count,
ElfW(Addr)* base, ElfW(Addr)* load_bias) {
for (size_t i = 0; i < phdr_count; ++i) {
if (phdr_table[i].p_type == PT_PHDR) {
*load_bias = reinterpret_cast<ElfW(Addr)>(phdr_table) - phdr_table[i].p_vaddr;
*base = reinterpret_cast<ElfW(Addr)>(phdr_table) - phdr_table[i].p_offset;
return;
}
}
async_safe_fatal("Could not find a PHDR: broken executable?");
}
static ElfW(Addr) __attribute__((noinline))
__linker_init_post_relocation(KernelArgumentBlock& args,
ElfW(Addr) linker_addr,
@ -524,22 +528,17 @@ extern "C" ElfW(Addr) __linker_init(void* raw_args) {
__libc_init_sysinfo(args);
#endif
// AT_BASE is set to 0 in the case when linker is run by iself
// so in order to link the linker it needs to calcuate AT_BASE
// using information at hand. The trick below takes advantage
// of the fact that the value of linktime_addr before relocations
// are run is an offset and this can be used to calculate AT_BASE.
static uintptr_t linktime_addr = reinterpret_cast<uintptr_t>(&linktime_addr);
ElfW(Addr) linker_addr = reinterpret_cast<uintptr_t>(&linktime_addr) - linktime_addr;
#if defined(__clang_analyzer__)
// The analyzer assumes that linker_addr will always be null. Make it an
// unknown value so we don't have to mark N places with NOLINTs.
//
// (`+=`, rather than `=`, allows us to sidestep a potential "unused store"
// complaint)
linker_addr += reinterpret_cast<uintptr_t>(raw_args);
#endif
ElfW(Addr) linker_addr = args.getauxval(AT_BASE);
if (linker_addr == 0) {
// When the linker is run by itself (rather than as an interpreter for
// another program), AT_BASE is 0. In that case, the AT_PHDR and AT_PHNUM
// aux values describe the linker, so use the phdr to find the linker's
// base address.
ElfW(Addr) load_bias;
get_elf_base_from_phdr(
reinterpret_cast<ElfW(Phdr)*>(args.getauxval(AT_PHDR)), args.getauxval(AT_PHNUM),
&linker_addr, &load_bias);
}
ElfW(Ehdr)* elf_hdr = reinterpret_cast<ElfW(Ehdr)*>(linker_addr);
ElfW(Phdr)* phdr = reinterpret_cast<ElfW(Phdr)*>(linker_addr + elf_hdr->e_phoff);