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authorJohannes Stoelp <johannes.stoelp@gmail.com>2024-04-06 22:53:33 +0200
committerJohannes Stoelp <johannes.stoelp@gmail.com>2024-04-06 22:53:33 +0200
commit3de8cfbc28ff4f7619c20d5552e03ca96aab4b01 (patch)
tree89258999dd265de301393151dd3e3b318d3c0e05 /src
parentc660b71b9689af89bc09671e6a6de7f9943e2709 (diff)
downloadnotes-3de8cfbc28ff4f7619c20d5552e03ca96aab4b01.tar.gz
notes-3de8cfbc28ff4f7619c20d5552e03ca96aab4b01.zip
ld.so: remove leading whitespaces
Diffstat (limited to 'src')
-rw-r--r--src/development/ld.so.md124
1 files changed, 62 insertions, 62 deletions
diff --git a/src/development/ld.so.md b/src/development/ld.so.md
index 062bac6..67ff6e9 100644
--- a/src/development/ld.so.md
+++ b/src/development/ld.so.md
@@ -2,13 +2,13 @@
## Environment Variables
```console
- LD_PRELOAD=<l_so> colon separated list of libso's to be pre loaded
- LD_DEBUG=<opts> comma separated list of debug options
- =help list available options
- =libs show library search path
- =files processing of input files
- =symbols show search path for symbol lookup
- =bindings show against which definition a symbol is bound
+LD_PRELOAD=<l_so> colon separated list of libso's to be pre loaded
+LD_DEBUG=<opts> comma separated list of debug options
+ =help list available options
+ =libs show library search path
+ =files processing of input files
+ =symbols show search path for symbol lookup
+ =bindings show against which definition a symbol is bound
```
### LD_LIBRARY_PATH and dlopen(3)
@@ -31,14 +31,14 @@ 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
-
- > LD_PRELOAD=liba.so:libb.so ./main
- -->
- preloaded in this order
- <--
- initialized in this order
+> ldd ./main
+ >> libc.so.6 => /usr/lib/libc.so.6
+
+> LD_PRELOAD=liba.so:libb.so ./main
+ -->
+ preloaded in this order
+ <--
+ initialized in this order
```
The preload order determines:
@@ -57,17 +57,17 @@ following:
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
-
- # init order
- >> calling init: /usr/lib/libc.so.6
- >> calling init: <path>/libb.so
- >> calling init: <path>/liba.so
- >> initialize program: ./main
+> 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
```
To verify the `link map` order we let `ld.so` resolve the `memcpy(3)` libc
@@ -75,12 +75,12 @@ symbol (used in _main_) dynamically, while enabling `LD_DEBUG=symbols,bindings`
to see the resolving in action.
```makrdown
- > 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]
+> 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)
@@ -91,36 +91,36 @@ 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
+.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
+[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
@@ -136,10 +136,10 @@ Using `radare2` we can analyze this in more detail:
- 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
- ```
+ > 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]`.