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WIP: smallest executable on Apple M1 (aarch64 or arm64)

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Requirements for success after snipping a loader_command:
    "codesign -s - my_app" must succeed.
    "lldb my_app; process launch -s; continue" must succeed.

Optional loader_commands (macho-snip can remove these successfully):
    LC_UUID, LC_BUILD_VERSION, LC_SOURCE_VERISON,
    LC_DATA_IN_CODE (when 0==datasize)

Apple "strip -N" clears out LC_SYMTAB and LC_DYSYMTAB, but leaves
LC_DYLD_INFO_ONLY.export_size.  Perhaps this could be zero if
constructed that way; snipping seems tedious because codesign
requires that __LINKEDIT must have no gaps.

LC_FUNCTION_STARTS seems to be required by codesign.
codesign wants offsets that point into __LINKEDIT to be in order:
    LC_DYLD_INFO_ONLY, LC_FUNCTION_STARTS, LC_DATA_IN_CODE, LC_SYMTAB

MacOS seesm to require LC_LOAD_DYLINKER (else "zsh: Kiled"),
which seems to require LD_LOAD_DYLIB (else SIGABRT).

https://github.com/upx/upx/issues/446
----
        modified:   macho-snip.c
	modified:   udf.s
This commit is contained in:
John Reiser 2022-03-01 15:06:58 -08:00 committed by Markus F.X.J. Oberhumer
parent 480ab51650
commit 252143d0bb
2 changed files with 25 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

38
src/stub/tools/macho-snip/macho-snip.c
View File

@ -247,10 +247,11 @@ main(int argc, char const * /*const*/ *const argv, char const *const *const envp
unsigned ncmds = mhdr->ncmds;
unsigned headway = mhdr->sizeofcmds;
struct load_command *cmd = (struct load_command *)(1+ mhdr);
for (; ncmds; --ncmds) {
struct load_command *cmd_next;
unsigned delta_dataoff = 0;
for (; ncmds; --ncmds, cmd = cmd_next) {
unsigned end_dataoff = 0;
unsigned end_datasize = 0;
struct load_command *cmd_next;
again: ;
fprintf(stderr, "cmd@%p %s %d(%#x)\n",
cmd, cmd_names[cmd->cmd&0xFF].name, cmd->cmd&0xFF, cmd->cmd);
@ -271,15 +272,7 @@ again: ;
}
} break;
case LC_CODE_SIGNATURE: {
fprintf(stderr, "macho-snip: LC_CODE_SIGNATURE not implemented\n");
continue;
struct linkedit_data_command *cmd_LED = (struct linkedit_data_command *)cmd;
if (( cmd_LED->dataoff + cmd_LED->datasize )
== (linkedit->fileoff + linkedit->filesize)) {
linkedit->filesize -= cmd_LED->datasize;
}
memset(addr + linkedit->fileoff, 0, cmdsize);
goto snip;
fprintf(stderr, "macho-snip: use 'codesign --remove-signature' to remove LC_CODE_SIGNATURE\n");
} break;
//struct nlist_64 {
@ -305,6 +298,8 @@ again: ;
// value per 4KB page) has offset (4 mod 16) instead of (0 mod 16).]
case LC_SYMTAB: {
fprintf(stderr, "macho-snip: LC_SYMTAB skipped\n");
continue;
struct symtab_command *symcmd = (struct symtab_command *)cmd;
if (( symcmd->strsize + symcmd->stroff)
!= (linkedit->filesize + linkedit->fileoff)) {
@ -328,6 +323,8 @@ again: ;
symcmd->nsyms -= 1; // lop last symbol FIXME: generalize
} break;
case LC_DYSYMTAB: {
fprintf(stderr, "macho-snip: LD_DYSYMTAB skipped\n");
continue;
struct dysymtab_command *dysym = (struct dysymtab_command *)cmd;
if (0==(dysym->nundefsym -= 1)) { // FIXME: generalize
dysym->iundefsym = 0;
@ -335,15 +332,25 @@ again: ;
} break;
case LC_BUILD_VERSION:
case LC_DYLD_INFO: // also LC_DYLD_INFO_ONLY because low 8 bits
case LC_LOAD_DYLIB:
case LC_LOAD_DYLINKER:
case LC_MAIN:
case LC_SOURCE_VERSION: {
case LC_SOURCE_VERSION:
case LC_UUID:
{
for (jargv = 2; jargv < argc; ++jargv) {
if (argv[jargv] && !strcmp(cmd_names[cmd->cmd & 0xFF].name, argv[jargv])) {
argv_done |= 1uL << jargv;
fprintf(stderr, "macho-snip: %#x, %s\n",
cmd_names[cmd->cmd & 0xFF].val, cmd_names[cmd->cmd & 0xFF].name);
// EXPERIMENT:
if (cmd->cmd == LC_DYLD_INFO_ONLY) { // the "must process" case
struct dyld_info_command *dyldcmd = (struct dyld_info_command *)cmd;
dyldcmd->export_off = 0;
dyldcmd->export_size = 0;
goto next; // EXPERIMENT
}
goto snip;
}
}
@ -366,8 +373,7 @@ again: ;
}
} break;
}
cmd = (struct load_command *)(cmdsize + (void *)cmd);
continue;
continue; // no changes ==> advance
snip_linkedit_data_command: ;
struct linkedit_data_command *ldc = (struct linkedit_data_command *)cmd;
end_datasize = ldc->datasize;
@ -379,12 +385,12 @@ snip_linkedit_data_command: ;
snip: ;
memmove(cmd, cmd_next, headway);
memset(headway + (char *)cmd, 0, cmdsize); // space that was vacated
cmd_next = cmd; // we moved *cmd_next to *cmd
cmd_next = cmd; // we moved tail at *cmd_next to *cmd
mhdr->sizeofcmds -= cmdsize;
mhdr->ncmds -= 1;
argv[jargv] = 0; // snip only once per argv[]
} // switch
cmd = cmd_next;
next: ;
} // ncmds
argv_done |= (1<<1) | (1<<0); // argv[0,1] do not name linker_commands
if (~(~0uL << argc) != argv_done) {

6
src/stub/tools/macho-snip/udf.s
View File

@ -1,9 +1,9 @@
.align 4
// start: .globl start # for standalone
_main: .globl _main # for -lc
// start: .globl start /* for standalone */
_main: .globl _main /* for -lc */
#ifdef __x86_64__
ud2
#endif
#ifdef __AARCH64EL__
.int 0x7b # udf 123
.int 0x7b /* udf 123 */
#endif