Keep argv[0], add COSMOPOLITAN_PROGRAM_EXECUTABLE (#980)

* Introduce env.com

Handy tool for debugging environment issues.

* Inject path as COSMOPOLITAN_PROGRAM_EXECUTABLE

`argv[0]` was previously being used as a communication channel between
the loader and the binary, giving the binary its full path for use e.g.
in `GetProgramExecutableName`. But `argv[0]` is not a good channel for
this; much of what made 2a3813c6 so gross is due to that.

This change fixes the issue by preserving `argv[0]` and establishing a
new communication channel: `COSMOPOLITAN_PROGRAM_EXECUTABLE`.

The M1 loader will always set this as the first variable. Linux should
soon follow. On the other side, `GetProgramExecutableName` checks that
variable first. If it sees it, it trusts it as-is.

A lot of the churn in `ape/ape-m1.c` in this change is actually backing
out hacks introduced in 2a3813c6; the best comparison is:

    git diff 2a3813c6^..

ape/ape-m1.c

@@ -16,6 +16,7 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include <assert.h>
 #include <dispatch/dispatch.h>
 #include <dlfcn.h>
 #include <errno.h>
@@ -35,6 +36,10 @@
 #include <unistd.h>
 
 #define pagesz         16384
+#define VARNAME        "COSMOPOLITAN_PROGRAM_EXECUTABLE="
+#define VARSIZE        (sizeof(VARNAME) - 1)
+/* maximum path size that cosmo can take */
+#define PATHSIZE       (PATH_MAX < 1024 ? PATH_MAX : 1024)
 #define SYSLIB_MAGIC   ('s' | 'l' << 8 | 'i' << 16 | 'b' << 24)
 #define SYSLIB_VERSION 8
 
@@ -198,8 +203,11 @@ struct PathSearcher {
   unsigned long namelen;
   const char *name;
   const char *syspath;
-  char path[1024];
+  char varname[VARSIZE];
+  char path[PATHSIZE];
 };
+_Static_assert(offsetof(struct PathSearcher, varname) + VARSIZE ==
+               offsetof(struct PathSearcher, path), "struct layout");
 
 struct ApeLoader {
   struct PathSearcher ps;
@@ -313,7 +321,14 @@ __attribute__((__noreturn__)) static void Pexit(const char *c, int failed,
 }
 
 static char AccessCommand(struct PathSearcher *ps, unsigned long pathlen) {
-  if (pathlen + 1 + ps->namelen + 1 > sizeof(ps->path)) return 0;
+  if (!pathlen && *ps->name != '/') {
+    if (!getcwd(ps->path, sizeof(ps->path) - 1 - ps->namelen)) {
+      Pexit("getcwd", -errno, "failed");
+    }
+    pathlen = strlen(ps->path);
+  } else if (pathlen + 1 + ps->namelen + 1 > sizeof(ps->path)) {
+    return 0;
+  }
   if (pathlen && ps->path[pathlen - 1] != '/') ps->path[pathlen++] = '/';
   memmove(ps->path + pathlen, ps->name, ps->namelen);
   ps->path[pathlen + ps->namelen] = 0;
@@ -884,8 +899,9 @@ int main(int argc, char **argv, char **envp) {
   struct ApeLoader *M;
   long *sp, *sp2, *auxv;
   union ElfEhdrBuf *ebuf;
-  int c, islogin, n, fd, rc;
-  char *p, *pe, *dash_l, *exe, *prog, *shell, *execfn;
+  int c, n, fd, rc;
+  char *p, *pe, *exe, *prog, *shell, *execfn;
+  char **varpos;
 
   /* allocate loader memory in program's arg block */
   n = sizeof(struct ApeLoader);
@@ -947,24 +963,15 @@ int main(int argc, char **argv, char **envp) {
   M->lib.dlclose = dlclose;
   M->lib.dlerror = dlerror;
 
-  /* 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. */
-  if ((islogin = argc > 0 && *argv[0] == '-' && (shell = GetEnv(envp, "SHELL"))
-       && !StrCmp(argv[0] + 1, BaseName(shell)))) {
-    execfn = shell;
-    dash_l = __builtin_alloca(3);
-    memmove(dash_l, "-l", 3);
-  } else {
-    execfn = argc > 0 ? argv[0] : 0;
-  }
-
   /* getenv("_") is close enough to at_execfn */
+  execfn = argc > 0 ? argv[0] : 0;
+  varpos = 0;
   for (i = 0; envp[i]; ++i) {
     if (envp[i][0] == '_' && envp[i][1] == '=') {
       execfn = envp[i] + 2;
+    } else if (!memcmp(VARNAME, envp[i], VARSIZE)) {
+      assert(!varpos);
+      varpos = envp + i;
     }
   }
 
@@ -975,24 +982,22 @@ int main(int argc, char **argv, char **envp) {
   /* 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 */
-  n = (auxv - sp + islogin + AUXV_WORDS + 1) * sizeof(long);
+  n = (auxv - sp + !varpos + 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 (islogin) {
-    memmove(sp2, sp, 2 * sizeof(long));
-    *((char **)sp2 + 2) = dash_l;
-    memmove(sp2 + 3, sp + 2, (auxv - sp - 2) * sizeof(long));
-    ++argc;
-    sp2[0] = argc;
-  } else {
-    memmove(sp2, sp, (auxv - sp) * sizeof(long));
-  }
+  memmove(sp2, sp, (auxv - sp) * sizeof(long));
 
   argv = (char **)(sp2 + 1);
   envp = (char **)(sp2 + 1 + argc + 1);
+  if (varpos) {
+    varpos = (char **)((long *)varpos - sp + sp2);
+  } else {
+    varpos = envp + i++;
+    *(envp + i) = 0;
+  }
   auxv = (long *)(envp + i + 1);
   sp = sp2;
 
@@ -1008,13 +1013,14 @@ int main(int argc, char **argv, char **envp) {
        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];
+    if (*argv[0] == '-' && (shell = GetEnv(envp, "SHELL")) &&
+        !StrCmp(argv[0] + 1, BaseName(shell))) {
+      execfn = prog = shell;
+    } else {
+      prog = (char *)sp[1];
+    }
     argc = sp[0];
     argv = (char **)(sp + 1);
-    if (islogin) {
-      ++argv[0];
-      prog = shell;
-    }
   } else if ((M->ps.literally = argc >= 3 && !StrCmp(argv[1], "-"))) {
     /* if the first argument is a hyphen then we give the user the
        power to change argv[0] or omit it entirely. most operating
@@ -1056,11 +1062,11 @@ int main(int argc, char **argv, char **envp) {
   }
   pe = ebuf->buf + rc;
 
-  /* resolve argv[0] to reflect path search */
-  if (argc > 0 && ((*prog != '/' && *exe == '/' && !StrCmp(prog, argv[0])) ||
-                   M->ps.indirect || !StrCmp(BaseName(prog), argv[0]))) {
-    argv[0] = exe;
-  }
+  /* inject program executable as first environment variable,
+     swapping the old first variable for it. */
+  memmove(M->ps.varname, VARNAME, VARSIZE);
+  *varpos = *envp;
+  *envp = M->ps.varname;
 
   /* generate some hard random data */
   if ((rc = sys_getentropy(M->rando, sizeof(M->rando))) < 0) {