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more docs!

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PoroCYon 1 year ago
parent
commit
d0291ec6cd
5 changed files with 157 additions and 32 deletions
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      README.md
  2. +3
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      rt/loader32.asm
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      rt/loader64.asm
  4. +4
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      smold.py
  5. +2
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      smoldd.py

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README.md View File

@ -17,15 +17,22 @@ PoC by Shiz, bugfixing and 64-bit version by PoroCYon.
`.text.startup._start`! Otherwise, the linker script will fail silently, and
the smol startup/symbol resolving code will jump to an undefined location.
***NOTE***: C++ exceptions, RTTI, global *external* variables, global
constructors and destructors (the ELF `.ctors`/`.dtors`/
`attribute((con-/destructor))` things, not the C++ language constructs), ...
aren't supported yet, and probably won't be anytime soon.
```sh
# example:
./smold.py -fuse-dnload-loader [--opts...] -lfoo -lbar input.o... output.elf
```
```
usage: smold.py [-h] [-m TARGET] [-l LIB] [-L DIR] [-s] [-c] [-n] [-d] [-fno-use-interp] [-fno-align-stack] [-fuse-nx] [-fuse-dnload-loader] [-fno-skip-zero-value] [-fuse-dt-debug] [-fuse-dl-fini] [-fskip-entries] [-fno-start-arg]
[-funsafe-dynamic] [-fno-ifunc-support] [-fifunc-strict-cconv] [--nasm NASM] [--cc CC] [--readelf READELF] [-Wc CFLAGS] [-Wa ASFLAGS] [-Wl LDFLAGS] [--smolrt SMOLRT] [--smolld SMOLLD] [--gen-rt-only] [--verbose]
[--keeptmp] [--debugout DEBUGOUT]
usage: smold.py [-h] [-m TARGET] [-l LIB] [-L DIR] [-s | -c] [-n] [-d] [-g] [-fuse-interp] [-falign-stack]
[-fskip-zero-value] [-fifunc-support] [-fuse-dnload-loader] [-fuse-nx] [-fuse-dt-debug]
[-fuse-dl-fini] [-fskip-entries] [-fno-start-arg] [-funsafe-dynamic] [-fifunc-strict-cconv]
[--nasm NASM] [--cc CC] [--readelf READELF] [-Wc CFLAGS] [-Wa ASFLAGS] [-Wl LDFLAGS]
[--smolrt SMOLRT] [--smolld SMOLLD] [--gen-rt-only] [--verbose] [--keeptmp] [--debugout DEBUGOUT]
input [input ...] output
positional arguments:
@ -39,26 +46,45 @@ optional arguments:
-l LIB, --library LIB
libraries to link against
-L DIR, --libdir DIR directories to search libraries in
-s, --hash16 Use 16-bit (BSD2) hashes instead of 32-bit djb2 hashes. Implies -fuse-dnload-loader. Only usable for 32-bit output.
-c, --crc32c Use Intel's crc32 intrinsic for hashing. Implies -fuse-dnload-loader. Conflicts with `--hash16'.
-n, --nx Use NX (i.e. don't use RWE pages). Costs the size of one phdr, plus some extra bytes on i386.
-s, --hash16 Use 16-bit (BSD2) hashes instead of 32-bit djb2 hashes. Implies -fuse-dnload-loader. Only
usable for 32-bit output.
-c, --crc32c Use Intel's crc32 intrinsic for hashing. Implies -fuse-dnload-loader. Conflicts with
`--hash16'.
-n, --nx Use NX (i.e. don't use RWE pages). Costs the size of one phdr, plus some extra bytes on
i386.
-d, --det Make the order of imports deterministic (default: just use whatever binutils throws at us)
-fno-use-interp Don't include a program interpreter header (PT_INTERP). If not enabled, ld.so has to be invoked manually by the end user.
-fno-align-stack Don't align the stack before running user code (_start). If not enabled, this has to be done manually. Frees 1 byte.
-fuse-nx Don't use one big RWE segment, but use separate RW and RE ones. Use this to keep strict kernels (PaX/grsec) happy. Costs at least the size of one program header entry.
-fuse-dnload-loader Use a dnload-style loader for resolving symbols, which doesn't depend on nonstandard/undocumented ELF and ld.so features, but is slightly larger. If not enabled, a smaller custom loader is used which assumes
glibc.
-fno-skip-zero-value Don't skip an ELF symbol with a zero address (a weak symbol) when parsing libraries at runtime. Try enabling this if you're experiencing sudden breakage. However, many libraries don't use weak symbols, so this
doesn't often pose a problem. Frees ~5 bytes.
-fuse-dt-debug Use the DT_DEBUG Dyn header to access the link_map, which doesn't depend on nonstandard/undocumented ELF and ld.so features. If not enabled, the link_map is accessed using data leaked to the entrypoint by ld.so,
which assumes glibc. Costs ~10 bytes.
-fuse-dl-fini Pass _dl_fini to the user entrypoint, which should be done to properly comply with all standards, but is very often not needed at all. Costs 2 bytes.
-fskip-entries Skip the first two entries in the link map (resp. ld.so and the vDSO). Speeds up symbol resolving, but costs ~5 bytes.
-fno-start-arg Don't pass a pointer to argc/argv/envp to the entrypoint using the standard calling convention. This means you need to read these yourself in assembly if you want to use them! (envp is a preprequisite for X11,
because it needs $DISPLAY.) Frees 3 bytes.
-funsafe-dynamic Don't end the ELF Dyn table with a DT_NULL entry. This might cause ld.so to interpret the entire binary as the Dyn table, so only enable this if you're sure this won't break things!
-fno-ifunc-support Support linking to IFUNCs. Probably needed on x86_64, but costs ~16 bytes. Ignored on platforms without IFUNC support.
-fifunc-strict-cconv On i386, if -fifunc-support is specified, strictly follow the calling convention rules. Probably not needed, but you never know.
-g, --debug Pass `-g' to the C compiler, assembler and linker. Only useful when `--debugout' is
specified.
-fuse-interp [Default ON] Include a program interpreter header (PT_INTERP). If not enabled, ld.so has to
be invoked manually by the end user. Disable with `-fno-use-interp'.
-falign-stack [Default ON] Align the stack before running user code (_start). If not enabled, this has to
be done manually. Costs 1 byte. Disable with `-fno-align-stack'.
-fskip-zero-value [Default: ON if `-fuse-dnload-loader' supplied, OFF otherwise] Skip an ELF symbol with a
zero address (a weak symbol) when parsing libraries at runtime. Try enabling this if you're
experiencing sudden breakage. However, many libraries don't use weak symbols, so this
doesn't often pose a problem. Costs ~5 bytes.Disable with `-fno-skip-zero-value'.
-fifunc-support [Default ON] Support linking to IFUNCs. Probably needed on x86_64, but costs ~16 bytes.
Ignored on platforms without IFUNC support. Disable with `-fno-fifunc-support'.
-fuse-dnload-loader Use a dnload-style loader for resolving symbols, which doesn't depend on
nonstandard/undocumented ELF and ld.so features, but is slightly larger. If not enabled, a
smaller custom loader is used which assumes glibc. `-fskip-zero-value' defaults to ON if
this flag is supplied.
-fuse-nx Don't use one big RWE segment, but use separate RW and RE ones. Use this to keep strict
kernels (PaX/grsec) happy. Costs at least the size of one program header entry.
-fuse-dt-debug Use the DT_DEBUG Dyn header to access the link_map, which doesn't depend on
nonstandard/undocumented ELF and ld.so features. If not enabled, the link_map is accessed
using data leaked to the entrypoint by ld.so, which assumes glibc. Costs ~10 bytes.
-fuse-dl-fini Pass _dl_fini to the user entrypoint, which should be done to properly comply with all
standards, but is very often not needed at all. Costs 2 bytes.
-fskip-entries Skip the first two entries in the link map (resp. ld.so and the vDSO). Speeds up symbol
resolving, but costs ~5 bytes.
-fno-start-arg Don't pass a pointer to argc/argv/envp to the entrypoint using the standard calling
convention. This means you need to read these yourself in assembly if you want to use them!
(envp is a preprequisite for X11, because it needs $DISPLAY.) Frees 3 bytes.
-funsafe-dynamic Don't end the ELF Dyn table with a DT_NULL entry. This might cause ld.so to interpret the
entire binary as the Dyn table, so only enable this if you're sure this won't break things!
-fifunc-strict-cconv On i386, if -fifunc-support is specified, strictly follow the calling convention rules.
Probably not needed, but you never know.
--nasm NASM which nasm binary to use
--cc CC which cc binary to use (MUST BE GCC!)
--readelf READELF which readelf binary to use
@ -70,10 +96,13 @@ optional arguments:
Flags to pass to the linker for the final linking step
--smolrt SMOLRT Directory containing the smol runtime sources
--smolld SMOLLD Directory containing the smol linker scripts
--gen-rt-only Only generate the headers/runtime assembly source file, instead of doing a full link. (I.e. fall back to pre-release behavior.)
--gen-rt-only Only generate the headers/runtime assembly source file, instead of doing a full link. (I.e.
fall back to pre-release behavior.)
--verbose Be verbose about what happens and which subcommands are invoked
--keeptmp Keep temp files (only useful for debugging)
--debugout DEBUGOUT Write out an additional, unrunnable debug ELF file with symbol information. (Useful for debugging with gdb, cannot be ran due to broken relocations.)
--debugout DEBUGOUT Write out an additional, unrunnable debug ELF file with symbol information. (Useful for
debugging with gdb, cannot be ran due to broken relocations.)
--hang-on-startup Hang on startup until a debugger breaks the code out of the loop. Only useful for debugging.
```
A minimal crt (and `_start` funcion) are provided in case you want to use `main`.
@ -86,7 +115,7 @@ during dynamic linking. (Think of it as an equivalent of `ldd`, except that it
also checks whether the imported functions are present as well.)
```
usage: smoldd.py [-h] [--cc CC] [--readelf READELF] [--map MAP] input
usage: smoldd.py [-h] [--cc CC] [--readelf READELF] [--map MAP] [-s | -c] input
positional arguments:
input input file
@ -95,7 +124,94 @@ optional arguments:
-h, --help show this help message and exit
--cc CC C compiler binary
--readelf READELF readelf binary
--map MAP Get the address of the symbol hash table from the linker map output instead of attempting to parse the binary.
--map MAP Get the address of the symbol hash table from the linker map
output instead of attempting to parse the binary.
-s, --hash16 Use 16-bit (BSD2) hashes instead of 32-bit djb2 hashes. Only
usable for 32-bit output.
-c, --crc32c Use Intel's crc32 intrinsic for hashing. Conflicts with `--hash16'.
```
## Debugging your smol-ified executable
So suddenly the output binaries are crashing, while non-smol-ified executables
run just fine. What could've happened?
First of all, it could be PEBCAK: are you compiling with the exact same set of
compiler flags for the optimized and the regular builds? There could always be
a broken codepath in the former.
Secondly, did you enable any of the "evil" flags that can possibly break
compatiblity, such as `-fno-use-interp`, `-fno-align-stack`,
`-fno-skip-zero-value`, `-fno-ifunc-support`, `-fno-start-arg`,
`-funsafe-dynamic`, etc.? Try disabling these first, or try specifying
`-fskip-zero-value`, `-fifunc-strict-cconv`, `-fuse-nx`, `-fuse-dt-debug`,
`-fuse-dl-fini` or `-fuse-dnload-loader` (or remove the last one if you already
were using it). If you had to enable `-fuse-dt-debug` or mess with
`-fuse-dnload-loader`, please file an issue. If you had to specify `-fuse-nx`,
please don't use PaX/grsec for democoding.
But let's assume smol is the cause of the issue here. The first thing you
should do, is to check whether the crash happens in smol's runtime linking
code, or your actual executable code. This can be done by adding an `int3`
(x86) or `bkpt` (ARM) instruction or `__builtin_trap()` intrinsic (GCC/clang)
at the very beginning of your `_start` function. If the binary is now exiting
with a `Trace/breakpoint trap` or `Undefined instruction` error (or something
similar) instead of a `Segmentation fault`, it means the segfault is happening
after smol's runtime linker code has ran.
### The error is happening in the smol runtime linking/startup code
A common source of crashes here is that a symbol actually might not have been
resolved correctly. Try checking the output of `smoldd.py`.
If that isn't the cause, it's time to dig out GDB (see a later section), find
out what roughly is going wrong, and send in an issue ticket.
### The error is happening after the smol runtime linking/startup code
If a segfault is happening here, it's most likely happening when the binary
tries to call an external function. One cause if this can be bad stack
alignment (try messing with `-f[no-]align-stack`, or fix your `_start` code).
Another is that a symbol might have a 'value' (relative address) of zero, which
means the function call turned into a jump to the ELF header of the library,
instead of to the actual function. In this case, try specifying the
`-fskip-zero-value` flag.
Of course, it's still entirely possible it's a yet-unknown calling convention,
reloction, or other issue. If it isn't one of the above known causes, it's yet
again time to dust off your GDB skills and open an issue.
### Attaching GDB to a smol-ified executable
As you might have noticed, GDB cannot run smol-ified executables by itself, as
the ELF headers are too messed up. However, the Linux kernel and glibc dynamic
linker are able to parse it just fine. This means you'll have to attach a live
process, ideally before it segfaults. As racing a below-one-millisecond
timeframe is difficult, there is another solution: specify the `--hang-on-startup`
flag. Then attach your (currently-stuck) process to GDB, increase the program
counter manually to break out of the infinite loop, then continue debugging as
usual.
However, here you don't have any symbols available (let alone DWARF source
info), which makes debugging a bit hard. This can be mitigated by specifying
the `-g` flag, and loading the file specified by the `--debugout` flag into gdb,
which will provide you with symbol and (if `-g` was specified) debugging info.
A quick overview:
```sh
python3 ./smold.py -g --hang-on-startup --debugout=path/to/out.smol.dbg \
[usual args...] input... path/to/out.smol
path/to/out.smol # run it (it will hang)
^Z # background the hung process
# 1. attach the backgrounded process
# 2. break out of the loop (x86_64 example, s/rip/eip/g for i386)
# 3. load symbol and debugging info
gdb -ex "attach $(jobs sp)" \
-ex 'set $rip=$rip+2' \
path/to/out.smol.dbg
```
## Internal workings


+ 3
- 1
rt/loader32.asm View File

@ -51,7 +51,9 @@ _smol_start:
push _symbols
%endif
;.loopme: jmp short .loopme
%ifdef HANG_ON_STARTUP
.loopme: jmp short .loopme
%endif
%ifdef USE_DNLOAD_LOADER
push eax
pop ebp


+ 6
- 2
rt/loader64.asm View File

@ -55,7 +55,9 @@ _smol_start:
pop r11
pop rdi
;.loopme: jmp short .loopme ; debugging
%ifdef HANG_ON_STARTUP
.loopme: jmp short .loopme ; debugging
%endif
.next_hash:
mov r14d, dword [rdi]
; assume it's nonzero
@ -240,7 +242,9 @@ repne scasd ; technically, scasq should be used, but meh. this is 1 byte smaller
pop r11
pop rdi
;.loopme: jmp short .loopme ; debugging
%ifdef HANG_ON_STARTUP
.loopme: jmp short .loopme ; debugging
%endif
.next_hash:
mov r14d, dword [rdi]
; assume we need at least one function


+ 4
- 0
smold.py View File

@ -137,6 +137,9 @@ def main():
help="Write out an additional, unrunnable debug ELF file with symbol "+\
"information. (Useful for debugging with gdb, cannot be ran due "+\
"to broken relocations.)")
parser.add_argument('--hang-on-startup', default=False, action='store_true', \
help="Hang on startup until a debugger breaks the code out of the "+\
"loop. Only useful for debugging.")
parser.add_argument('input', nargs='+', help="input object file")
parser.add_argument('output', type=str, help="output binary")
@ -168,6 +171,7 @@ def main():
if args.falign_stack: args.asflags.insert(0, "-DALIGN_STACK")
if args.fifunc_support: args.asflags.insert(0, "-DIFUNC_SUPPORT")
if args.fifunc_strict_cconv: args.asflags.insert(0, "-DIFUNC_CORRECT_CCONV")
if args.hang_on_startup: args.asflags.insert(0, "-DHANG_ON_STARTUP")
for x in ['nasm','cc','readelf']:
val = args.__dict__[x]


+ 2
- 3
smoldd.py View File

@ -157,10 +157,9 @@ def main():
hashgrp = parser.add_mutually_exclusive_group()
hashgrp.add_argument('-s', '--hash16', default=False, action='store_true', \
help="Use 16-bit (BSD2) hashes instead of 32-bit djb2 hashes. "+\
"Implies -fuse-dnload-loader. Only usable for 32-bit output.")
"Only usable for 32-bit output.")
hashgrp.add_argument('-c', '--crc32c', default=False, action='store_true', \
help="Use Intel's crc32 intrinsic for hashing. "+\
"Implies -fuse-dnload-loader. Conflicts with `--hash16'.")
help="Use Intel's crc32 intrinsic for hashing. Conflicts with `--hash16'.")
args = parser.parse_args()
blob = args.input.read()


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