ape loaders modify argv[0] for older binaries

This repurposes one of the ELF padding bytes for a flag that is always 1 for modern ape binaries. If it is set, then the loader can safely assume that the binary knows about the register for program_executable_name. If it is not set, then the loader will unconditionally reset argv[0] to the located exe. This makes preserve-argv[0] backwards-compatible with older binaries.
jart · May 6, 2024 · bf6206f · bf6206f
1 parent a632695
commit bf6206f
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Showing 3 changed files with 14 additions and 3 deletions.
diff --git a/ape/ape-m1.c b/ape/ape-m1.c
@@ -124,6 +124,7 @@ struct Syslib {
 #define PT_INTERP                   3
 #define EI_CLASS                    4
 #define EI_DATA                     5
+#define EI_APE_FLAGS                9
 #define PF_X                        1
 #define PF_W                        2
 #define PF_R                        4
@@ -799,7 +800,7 @@ __attribute__((__noreturn__)) static void Spawn(const char *exe, int fd,
 }
 
 static const char *TryElf(struct ApeLoader *M, union ElfEhdrBuf *ebuf,
-                          const char *exe, int fd, long *sp, long *auxv,
+                          char *exe, int fd, long *sp, long *auxv,
                           char *execfn) {
   long i, rc;
   unsigned size;
@@ -814,6 +815,10 @@ static const char *TryElf(struct ApeLoader *M, union ElfEhdrBuf *ebuf,
   if (e->e_ident[EI_CLASS] == ELFCLASS32) {
     return "32-bit ELF isn't supported";
   }
+  if (!e->e_ident[EI_APE_FLAGS]) {
+    /* change argv[0] to resolved path */
+    ((char **)(sp + 1))[0] = exe;
+  }
   if (e->e_type != ET_EXEC && e->e_type != ET_DYN) {
     return "ELF not ET_EXEC or ET_DYN";
   }

diff --git a/ape/ape.S b/ape/ape.S
@@ -661,8 +661,9 @@ apesh:	.ascii	"\n@\n#'\"\n"			// sixth edition shebang
 	.ascii	    "\\1"			//   6: elf v1.o
 	.ascii	    "\\011"			//   7: FreeBSD
 	.ascii	    "\\0"			//   8: os/abi ver.
-	.ascii	    "\\0\\0\\0"			//   9: padding 3/7
-	.ascii	    "\\0\\0\\0\\0"		//      padding 4/7
+	.ascii	    "\\1"			//   9: ape flags
+	.ascii	    "\\0\\0"			//      padding 2/6
+	.ascii	    "\\0\\0\\0\\0"		//      padding 4/6
 	.ascii	    "\\2\\0"			//  10: εxεcµταblε
 	.ascii	    "\\076\\0"			//  12: NexGen32e
 	.ascii	    "\\1\\0\\0\\0"		//  14: elf v1.o

diff --git a/ape/loader.c b/ape/loader.c
@@ -134,6 +134,7 @@
 #define PT_INTERP                   3
 #define EI_CLASS                    4
 #define EI_DATA                     5
+#define EI_APE_FLAGS                9
 #define PF_X                        1
 #define PF_W                        2
 #define PF_R                        4
@@ -822,6 +823,10 @@ static const char *TryElf(struct ApeLoader *M, union ElfEhdrBuf *ebuf,
   if (e->e_ident[EI_CLASS] == ELFCLASS32) {
     return "32-bit ELF isn't supported";
   }
+  if (!e->e_ident[EI_APE_FLAGS]) {
+    /* change argv[0] to resolved path */
+    ((char **)(sp + 1))[0] = exe;
+  }
   if (e->e_type != ET_EXEC && e->e_type != ET_DYN) {
     return "ELF not ET_EXEC or ET_DYN";
   }