Loaders rewrite argv[0] for old binaries (#1170)

For this to work, a loader has to be able to tell the difference between
an ‘old’ and a ‘new’ binary. This is achieved via a repurposing of ELF’s
e_flags field. We previously tried to use the padding in e_ident for it,
but binutils was resetting it to zero in e.g. strip.

This introduces one new ELF flag for cosmopolitan binaries. It is called
`EF_APE_MODERN`. We choose 0x101ca75, "lol cat 5".

It should now be safe to install the ape loader binfmt registration with
the `P` flag.

ape/ape-m1.c

@@ -146,6 +146,9 @@ struct Syslib {
 #define AT_RANDOM                   25
 #define AT_EXECFN                   31
 
+#define EF_APE_MODERN      0x101ca75
+#define EF_APE_MODERN_MASK 0x1ffffff
+
 #define AUXV_WORDS 31
 
 /* from the xnu codebase */
@@ -799,7 +802,7 @@ __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;
@@ -820,6 +823,10 @@ static const char *TryElf(struct ApeLoader *M, union ElfEhdrBuf *ebuf,
   if (e->e_machine != EM_AARCH64) {
     return "couldn't find ELF header with ARM64 machine type";
   }
+  if ((e->e_flags & EF_APE_MODERN_MASK) != EF_APE_MODERN && sp[0] > 0) {
+    /* change argv[0] to resolved path for older binaries */
+    ((char **)(sp + 1))[0] = exe;
+  }
   if (e->e_phentsize != sizeof(struct ElfPhdr)) {
     Pexit(exe, 0, "e_phentsize is wrong");
   }

ape/ape.S

@@ -196,7 +196,7 @@ ape_mz:
 	.quad	ape_elf_entry			//  18: e_entry
 	.quad	ape_elf_phoff			//  20: e_phoff
 	.quad	ape_elf_shoff			//  28: e_shoff
-	.long	0				//  30: e_flags
+	.long	0x101ca75			//  30: ape e_flags
 	.short	64				//  34: e_ehsize
 	.short	56				//  36: e_phentsize
 	.short	ape_elf_phnum			//  38: e_phnum
@@ -669,7 +669,7 @@ apesh:	.ascii	"\n@\n#'\"\n"			// sixth edition shebang
 	.shstub	    ape_elf_entry,8		//  18: e_entry
 	.shstub	    ape_elf_phoff,8		//  20: e_phoff
 	.shstub	    ape_elf_shoff,8		//  28: e_shoff
-	.ascii	    "\\0\\0\\0\\0"		//  30: e_flags
+	.ascii	    "\\165\\312\\1\\1"		//  30: ape e_flags
 	.ascii	    "\\100\\0"			//  34: e_ehsize
 	.ascii	    "\\070\\0"			//  36: e_phentsize
 	.shstub	    ape_elf_phnum,2		//  38: e_phnum

ape/loader.c

@@ -152,6 +152,9 @@
 #define PR_SET_MM                   35
 #define PR_SET_MM_EXE_FILE          13
 
+#define EF_APE_MODERN      0x101ca75
+#define EF_APE_MODERN_MASK 0x1ffffff
+
 #define READ32(S)                                                      \
   ((unsigned)(255 & (S)[3]) << 030 | (unsigned)(255 & (S)[2]) << 020 | \
    (unsigned)(255 & (S)[1]) << 010 | (unsigned)(255 & (S)[0]) << 000)
@@ -834,6 +837,10 @@ static const char *TryElf(struct ApeLoader *M, union ElfEhdrBuf *ebuf,
     return "couldn't find ELF header with x86-64 machine type";
   }
 #endif
+  if ((e->e_flags & EF_APE_MODERN_MASK) != EF_APE_MODERN && sp[0] > 0) {
+    /* change argv[0] to resolved path for older binaries */
+    ((char **)(sp + 1))[0] = exe;
+  }
   if (e->e_phentsize != sizeof(struct ElfPhdr)) {
     Pexit(os, exe, 0, "e_phentsize is wrong");
   }

libc/elf/def.h

@@ -85,6 +85,10 @@
 #define EM_RISCV     243
 #define EM_BPF       247
 
+/* the ape flag, "lol cat 5" */
+#define EF_APE_MODERN      0x101ca75
+#define EF_APE_MODERN_MASK 0x1ffffff
+
 #define GRP_COMDAT 1
 #define STN_UNDEF  0
 

tool/build/fixupobj.c

@@ -327,6 +327,11 @@ static void UseFreebsdOsAbi(void) {
   elf->e_ident[EI_OSABI] = ELFOSABI_FREEBSD;
 }
 
+static void WriteApeFlags(void) {
+  /* try to be forward-compatible */
+  elf->e_flags = (elf->e_flags & ~EF_APE_MODERN_MASK) | EF_APE_MODERN;
+}
+
 /**
  * Improve GCC11 `-fpatchable-function-entry` codegen.
  *
@@ -668,6 +673,7 @@ static void FixupObject(void) {
           UseFreebsdOsAbi();
       }
       if (elf->e_type != ET_REL) {
+        WriteApeFlags();
         PurgeIfuncSections();
         RelinkZipFiles();
       }