Docs, safer -l flag injection

ape/ape-m1.c

@@ -882,10 +882,10 @@ static const char *TryElf(struct ApeLoader *M, union ElfEhdrBuf *ebuf,
 
 int main(int argc, char **argv, char **envp) {
   unsigned i;
-  int c, n, fd, rc;
   struct ApeLoader *M;
   long *sp, *sp2, *auxv;
   union ElfEhdrBuf *ebuf;
+  int c, islogin, n, fd, rc;
   char *p, *pe, *exe, *prog, *shell, *execfn;
 
   /* allocate loader memory in program's arg block */
@@ -960,18 +960,25 @@ int main(int argc, char **argv, char **envp) {
   sp = (long *)(argv - 1);
   auxv = (long *)(envp + i + 1);
 
+  /* there is a common convention of shells being told that they
+     are login shells via the OS prepending a - to their argv[0].
+     the APE system doesn't like it when argv[0] is not the full
+     path of the binary. to rectify this, the loader puts a "-l"
+     flag in argv[1] and ignores the dash. */
+  islogin = argc > 0 && *argv[0] == '-' && (shell = GetEnv(envp, "SHELL")) &&
+    !StrCmp(argv[0] + 1, BaseName(shell));
+
   /* create new bottom of stack for spawned program
      system v abi aligns this on a 16-byte boundary
      grows down the alloc by poking the guard pages */
 
-  c = argc > 0 && *argv[0] == '-';
-  n = (auxv - sp + c + AUXV_WORDS + 1) * sizeof(long);
+  n = (auxv - sp + islogin + AUXV_WORDS + 1) * sizeof(long);
   sp2 = (long *)__builtin_alloca(n);
   if ((long)sp2 & 15) ++sp2;
   for (; n > 0; n -= pagesz) {
     ((char *)sp2)[n - 1] = 0;
   }
-  if (c) {
+  if (islogin) {
     memmove(sp2, sp, 2 * sizeof(long));
     memmove(M->dash_l, "-l", 3);
     *((char **)sp2 + 2) = M->dash_l;
@@ -991,14 +998,19 @@ int main(int argc, char **argv, char **envp) {
   if ((M->ps.indirect = argc > 0 ? GetIndirectOffset(argv[0]) : 0)) {
     M->ps.literally = 0;
     /* if argv[0] is $prog.ape, then we strip off the .ape and run
-       $prog. */
+       $prog. This allows you to use symlinks to trick the OS when
+       a native executable is required. For example, let's say you
+       want to use the APE binary /opt/cosmos/bin/bash as a system
+       shell in /etc/shells, or perhaps in shebang lines like e.g.
+       `#!/opt/cosmos/bin/bash`. That won't work with APE normally,
+       but it will if you say:
+           ln -sf /usr/local/bin/ape /opt/cosmos/bin/bash.ape
+       and then use #!/opt/cosmos/bin/bash.ape instead. */
     prog = (char *)sp[1];
     argc = sp[0];
     argv = (char **)(sp + 1);
-    /* when we are started as a login shell, a '-' is prepended to
-       our name. TODO: indicate this to the loaded process somehow. */
-    if (*prog == '-' && ((shell = GetEnv(envp, "SHELL")) &&
-                         !StrCmp(BaseName(shell), prog + 1))) {
+    if (islogin) {
+      ++argv[0];
       prog = shell;
     }
   } else if ((M->ps.literally = argc >= 3 && !StrCmp(argv[1], "-"))) {
@@ -1042,15 +1054,9 @@ int main(int argc, char **argv, char **envp) {
   }
   pe = ebuf->buf + rc;
 
-  if (M->ps.indirect) {
-    prog = M->ps.path;
-    argv[0][M->ps.indirect] = 0;
-    if (*argv[0] == '-' && *BaseName(exe) != '-') ++argv[0];  /* XXX */
-  }
-
   /* resolve argv[0] to reflect path search */
   if (argc > 0 && ((*prog != '/' && *exe == '/' && !StrCmp(prog, argv[0])) ||
-                   !StrCmp(BaseName(prog), argv[0]))) {
+                   M->ps.indirect || !StrCmp(BaseName(prog), argv[0]))) {
     argv[0] = exe;
   }