add an option for vfork(2) support

README.md

@@ -20,8 +20,7 @@ but even smaller. It doesn't have a 64-bit version, though.**
 | Min. platform | Linux 3.19 | Linux 2.27 | Linux 2.27 | Most Unices |
 | Touches filesystem | N |       N |         N |                 Y |
 
-\*: with compatibility for systems where `/bin` isn't symlinked to `/usr/bin`
-(or the other way around).
+\*: with `NO_UBUNTU_COMPAT` **dis**abled.
 
 The exact size of a shell-based unpacker depends on the exact impmelentation,
 many variations exist. 'xz' means the usage of `xzcat` instead of `zcat`,
@@ -36,16 +35,37 @@ kernel is 64-bit and supports the 32-bit emulation layer.
 ## Usage
 
 ```
-nasm -fbin -o$out vondehi.asm [-DUSE_GZIP] [-DTAG="j0!"] [-DNO_UBUNTU_COMPAT]
+nasm -fbin -o$out vondehi.asm [-DUSE_GZIP] [-DTAG="j0!"] [-DNO_UBUNTU_COMPAT] \
+    [-DUSE_VFORK]
 cat $out $intro_compressed > $final
 ```
 
-By default, `xz` is used. A tag can be supplied to include a vanity string
-in the unpacker itself, right before the data.
+### Settings
+
+* `USE_GZIP` (default off): use `gzip` (`/bin/zcat`) instead of `xz`
+  (`/usr/bin/xzcat`).
+* `NO_UBUNTU_COMPAT` (default off): assume `/bin` is the same as `/usr/bin`.
+  Originally named like this because on my machine, `/bin` is linked t
+  `/usr/bin`, but on the Revision compomachine (which runs Ubuntu), it isn't.
+* `USE_VFORK` (default off): use `vfork(2)` instead of `fork(2)`. I hope you
+  know what you're doing when you enable this.
+* `TAG` (default empty): add a vanity tag right before the compressed data.
+  Only use this when you have bytes to spare, of course.
+
+## How to debug it if it doesn't work
+
+1. `strace` it
+2. See where errors start happening
+  * This can be obscured because the code assumes eg. syscall return values to
+    be between `0` and `255`, so later syscalls might fail, or nonsense
+    syscalls might be invoked.
+3. Fix it. Somehow.
 
 ## Greets to
 
-* Blackle, for the original Fishypack
+* Blackle, for the original Fishypack, and for replacing the `waitid(2)` call
+  with `waitpid(2)`, fixing compatibility with some kernels and shaving off a
+  few bytes at once!
 * Shiz, for other packing/unpacking and x86-related stuff
 * Faemiyah, yx, etc., for small sh-based unpackers (yx: nice trick with
   the script partially embedded in the gzip file!)

vondehi.asm

@@ -18,6 +18,7 @@
 %define SYS_lseek			19
 %define SYS_dup2			63
 %define SYS_execveat		358
+%define SYS_vfork			190
 
 %define EBP_bias			((_start-ehdr)-(__strempty-__self))
 
@@ -79,7 +80,11 @@ _start.2:
 	;dd filesize	;~p_memsz
 	db 5			;~p_flags
 _start.3:
+%ifdef USE_VFORK
+	mov al, SYS_vfork
+%else
 	mov al, SYS_fork
+%endif
 	jmp short _start.4
 
 	;dd 5			;~p_flags