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Diffstat (limited to 'src/ld.so.md')
| -rw-r--r-- | src/ld.so.md | 185 |
1 files changed, 104 insertions, 81 deletions
diff --git a/src/ld.so.md b/src/ld.so.md index 6dc8c49..311d1be 100644 --- a/src/ld.so.md +++ b/src/ld.so.md @@ -1,9 +1,9 @@ # ld.so(8) -## Environment variables +## Environment Variables ```console LD_PRELOAD=<l_so> colon separated list of libso's to be pre loaded - LD_DEBUG=<opts> comman separated list of debug options + LD_DEBUG=<opts> comma separated list of debug options =help list available options =libs show library search path =files processing of input files @@ -11,7 +11,10 @@ =bindings show against which definition a symbol is bound ``` -## LD_PRELOAD load & init order +## LD_PRELOAD: Initialization Order and Link Map +Libraries specified in `LD_PRELOAD` are loaded from `left-to-right` but +initialized from `right-to-left`. + ```markdown > ldd ./main >> libc.so.6 => /usr/lib/libc.so.6 @@ -21,87 +24,107 @@ preloaded in this order <-- initialized in this order +``` + +The preload order determines: +- the order libraries are inserted into the `link map` +- the initialization order for libraries + +For the example listed above the resulting `link map` will look like the +following: + +```makrdown + +------+ +------+ +------+ +------+ + | main | -> | liba | -> | libb | -> | libc | + +------+ +------+ +------+ +------+ +``` + +This can be seen when running with `LD_DEBUG=files`: + +```makrdown + > LD_DEBUG=files LD_PRELOAD=liba.so:libb.so ./main + # load order (-> determines link map) + >> file=liba.so [0]; generating link map + >> file=libb.so [0]; generating link map + >> file=libc.so.6 [0]; generating link map - - preload order determines the order libs are inserted into the link map - - - resulting link map: - +------+ +------+ +------+ +------+ - | main | -> | liba | -> | libb | -> | libc | - +------+ +------+ +------+ +------+ - - - see preload and init order in action - > LD_DEBUG=files LD_PRELOAD=liba.so:libb.so ./main - # load order (-> determines link map) - >> file=liba.so [0]; generating link map - >> file=libb.so [0]; generating link map - >> file=libc.so.6 [0]; generating link map - - # init order - >> calling init: /usr/lib/libc.so.6 - >> calling init: <path>/libb.so - >> calling init: <path>/liba.so - >> initialize program: ./main - - - see the symbol lookup in action and therefore the link map order - > LD_DEBUG=symbols,bindings LD_PRELOAD=liba.so:libb.so ./main - >> symbol=memcpy; lookup in file=./main [0] - >> symbol=memcpy; lookup in file=<path>/liba.so [0] - >> symbol=memcpy; lookup in file=<path>/libb.so [0] - >> symbol=memcpy; lookup in file=/usr/lib/libc.so.6 [0] - >> binding file ./main [0] to /usr/lib/libc.so.6 [0]: normal symbol - `memcpy' [GLIBC_2.14] + # init order + >> calling init: /usr/lib/libc.so.6 + >> calling init: <path>/libb.so + >> calling init: <path>/liba.so + >> initialize program: ./main ``` +To verify the `link map` order we let `ld.so` resolve the `memcpy(3)` libc +symbol (used in _main_) dynamically, while enabling `LD_DEBUG=symbols,bindings` +to see the resolving in action. -## dynamic linking (x86_64) ```makrdown - - dynamic linking basically works via one indirect jump. It uses a - combination of function trampolines (.plt) and a function pointer table - (.got.plt). On the first call the trampoline sets up some metadata and - then jumps to the ld.so runtime resolve function, which in turn patches - the table with the correct function pointer. - .plt ....... contains function trampolines, usually located in code - segment (rx permission) - .got.plt ... hold the function pointer table - - - following r2 dump shows this - - [0x00401030] indirect jump for 'puts' using function pointer in - _GLOBAL_OFFSET_TABLE_[3] - - initially points to instruction behind 'puts' trampoline [0x00401036] - - this pushes relocation index and then jumps to the first trampoline - [0x00401020] - - the first trampoline jumps to _GLOBAL_OFFSET_TABLE_[2] which will be - filled at program startup by the ld.so with its resolve function - - the resolve function fixes the relocation referenced by the - relocation index pushed by the 'puts' trampoline - - the relocation entry tells the resolve function which symbol to - search for and where to put the function pointer - > readelf -r <main> - >> Relocation section '.rela.plt' at offset 0x4b8 contains 1 entry: - >> Offset Info Type Sym. Value Sym. Name + Addend - >> 000000404018 000200000007 R_X86_64_JUMP_SLO 0000000000000000 puts@GLIBC_2.2.5 + 0 - - offset points to _GLOBAL_OFFSET_TABLE_[3] - - [0x00401040]> pd 4 @ section..got.plt - ;-- section..got.plt: - ;-- .got.plt: ; [22] -rw- section size 32 named .got.plt - ;-- _GLOBAL_OFFSET_TABLE_: - 0x00404000 .qword 0x0000000000403e10 ; section..dynamic ; obj._DYNAMIC - 0x00404008 .qword 0x0000000000000000 - ; CODE XREF from section..plt @ +0x6 - 0x00404010 .qword 0x0000000000000000 - ;-- reloc.puts: - ; CODE XREF from sym.imp.puts @ 0x401030 - 0x00404018 .qword 0x0000000000401036 ; RELOC 64 puts - - [0x00401040]> pd 6 @ section..plt - ;-- section..plt: - ;-- .plt: ; [12] -r-x section size 32 named .plt - ┌─> 0x00401020 ff35e22f0000 push qword [0x00404008] - ╎ 0x00401026 ff25e42f0000 jmp qword [0x00404010] - ╎ 0x0040102c 0f1f4000 nop dword [rax] - ┌ 6: int sym.imp.puts (const char *s); - └ ╎ 0x00401030 ff25e22f0000 jmp qword [reloc.puts] - ╎ 0x00401036 6800000000 push 0 - └─< 0x0040103b e9e0ffffff jmp sym..plt + > LD_DEBUG=symbols,bindings LD_PRELOAD=liba.so:libb.so ./main + >> symbol=memcpy; lookup in file=./main [0] + >> symbol=memcpy; lookup in file=<path>/liba.so [0] + >> symbol=memcpy; lookup in file=<path>/libb.so [0] + >> symbol=memcpy; lookup in file=/usr/lib/libc.so.6 [0] + >> binding file ./main [0] to /usr/lib/libc.so.6 [0]: normal symbol `memcpy' [GLIBC_2.14] ``` + +## Dynamic Linking (x86_64) +Dynamic linking basically works via one indirect jump. It uses a combination of +function trampolines (`.plt` section) and a function pointer table (`.got.plt` +section). +On the first call the trampoline sets up some metadata and then jumps to the +`ld.so` runtime resolve function, which in turn patches the table with the +correct function pointer. +```makrdown + .plt ....... procedure linkage table, contains function trampolines, usually + located in code segment (rx permission) + .got.plt ... global offset table for .plt, holds the function pointer table +``` + +Using `radare2` we can analyze this in more detail: + +```makrdown + [0x00401040]> pd 4 @ section..got.plt + ;-- section..got.plt: + ;-- .got.plt: ; [22] -rw- section size 32 named .got.plt + ;-- _GLOBAL_OFFSET_TABLE_: + [0] 0x00404000 .qword 0x0000000000403e10 ; section..dynamic + [1] 0x00404008 .qword 0x0000000000000000 + ; CODE XREF from section..plt @ +0x6 + [2] 0x00404010 .qword 0x0000000000000000 + ;-- reloc.puts: + ; CODE XREF from sym.imp.puts @ 0x401030 + [3] 0x00404018 .qword 0x0000000000401036 ; RELOC 64 puts + + [0x00401040]> pd 6 @ section..plt + ;-- section..plt: + ;-- .plt: ; [12] -r-x section size 32 named .plt + ┌─> 0x00401020 ff35e22f0000 push qword [0x00404008] + ╎ 0x00401026 ff25e42f0000 jmp qword [0x00404010] + ╎ 0x0040102c 0f1f4000 nop dword [rax] + ┌ 6: int sym.imp.puts (const char *s); + └ ╎ 0x00401030 ff25e22f0000 jmp qword [reloc.puts] + ╎ 0x00401036 6800000000 push 0 + └─< 0x0040103b e9e0ffffff jmp sym..plt +``` + +- At address `0x00401030` in the `.plt` section we see the indirect jump for + `puts` using the function pointer in `_GLOBAL_OFFSET_TABLE_[3] (GOT)`. +- `GOT[3]` initially points to instruction after the `puts` trampoline + `0x00401036`. +- This pushes the relocation index `0` and then jumps to the first trampoline + `0x00401020`. +- The first trampoline jumps to `GOT[2]` which will be filled at program + startup by the `ld.so` with its resolve function. +- The `ld.so` resolve function fixes the relocation referenced by the + relocation index pushed by the `puts` trampoline. +- The relocation entry at index `0` tells the resolve function which symbol to + search for and where to put the function pointer: + ```makrdown + > readelf -r <main> + >> Relocation section '.rela.plt' at offset 0x4b8 contains 1 entry: + >> Offset Info Type Sym. Value Sym. Name + Addend + >> 000000404018 000200000007 R_X86_64_JUMP_SLO 0000000000000000 puts@GLIBC_2.2.5 + 0 + ``` + As we can see the offset from relocation at index `0` points to `GOT[3]`. + |