Support binfmt_misc P flag in APE loader (#1058)

This allows ape to automatically preserve `argv[0]` [as of Linux kernel
5.12][0] if the [binfmt_misc][1] registration contains the P flag.

This also removes may_path_search, which was identical to the literally
flag in usage. As a result, FindCommand is subsumed into Commandv.

[0]: https://patchew.org/QEMU/20210222105004.1642234-1-laurent@vivier.eu/
[1]: https://www.kernel.org/doc/html/latest/admin-guide/binfmt-misc.html

ape/loader.c

@@ -141,6 +141,9 @@
 #define AT_PHENT                    4
 #define AT_PHNUM                    5
 #define AT_PAGESZ                   6
+#define AT_FLAGS                    8
+#define AT_FLAGS_PRESERVE_ARGV0_BIT 0
+#define AT_FLAGS_PRESERVE_ARGV0     (1 << AT_FLAGS_PRESERVE_ARGV0_BIT)
 #define AT_EXECFN_LINUX             31
 #define AT_EXECFN_NETBSD            2014
 #define X_OK                        1
@@ -572,39 +575,15 @@ static char SearchPath(struct PathSearcher *ps) {
   }
 }
 
-static char FindCommand(struct PathSearcher *ps) {
-  ps->path[0] = 0;
-
-  /* paths are always 100% taken literally when a slash exists
-       $ ape foo/bar.com arg1 arg2 */
-  if (MemChr(ps->name, '/', ps->namelen)) {
-    return AccessCommand(ps, 0);
-  }
-
-  /* we don't run files in the current directory
-       $ ape foo.com arg1 arg2
-     unless $PATH has an empty string entry, e.g.
-       $ expert PATH=":/bin"
-       $ ape foo.com arg1 arg2
-     however we will execute this
-       $ ape - foo.com foo.com arg1 arg2
-     because cosmo's execve needs it */
-  if (ps->literally && AccessCommand(ps, 0)) {
-    return 1;
-  }
-
-  /* otherwise search for name on $PATH */
-  return SearchPath(ps);
-}
-
 static char *Commandv(struct PathSearcher *ps, int os, char *name,
-                      const char *syspath, int may_path_search) {
+                      const char *syspath) {
-  if (!may_path_search) return name;
+  if (!(ps->namelen = StrLen((ps->name = name)))) return 0;
+  if (ps->literally || MemChr(ps->name, '/', ps->namelen)) return name;
   ps->os = os;
   ps->syspath = syspath ? syspath : "/bin:/usr/local/bin:/usr/bin";
-  if (!(ps->namelen = StrLen((ps->name = name)))) return 0;
   if (ps->namelen + 1 > sizeof(ps->path)) return 0;
-  if (FindCommand(ps)) {
+  ps->path[0] = 0;
+  if (SearchPath(ps)) {
     return ps->path;
   } else {
     return 0;
@@ -913,10 +892,10 @@ EXTERN_C __attribute__((__noreturn__)) void ApeLoader(long di, long *sp,
                                                       char dl) {
   int rc, n;
   unsigned i;
-  char literally;
   const char *ape;
   int c, fd, os, argc;
   struct ApeLoader *M;
+  char arg0, literally;
   unsigned long pagesz;
   union ElfEhdrBuf *ebuf;
   long *auxv, *ap, *endp, *sp2;
@@ -961,11 +940,14 @@ EXTERN_C __attribute__((__noreturn__)) void ApeLoader(long di, long *sp,
 
   /* detect netbsd and find end of words */
   pagesz = 0;
+  arg0 = 0;
   for (ap = auxv; ap[0]; ap += 2) {
     if (ap[0] == AT_PAGESZ) {
       pagesz = ap[1];
     } else if (SupportsNetbsd() && !os && ap[0] == AT_EXECFN_NETBSD) {
       os = NETBSD;
+    } else if (SupportsLinux() && ap[0] == AT_FLAGS) {
+      arg0 = !!(ap[1] & AT_FLAGS_PRESERVE_ARGV0);
     }
   }
   if (!pagesz) {
@@ -979,8 +961,12 @@ EXTERN_C __attribute__((__noreturn__)) void ApeLoader(long di, long *sp,
   }
 
   /* we can load via shell, shebang, or binfmt_misc */
-  int may_path_search = 1;
+  if (arg0) {
-  if ((literally = argc >= 3 && !StrCmp(argv[1], "-"))) {
+    literally = 1;
+    prog = (char *)sp[2];
+    argc = sp[2] = sp[0] - 2;
+    argv = (char **)((sp += 2) + 1);
+  } else if ((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
        systems don't permit the omission of argv[0] but we do, b/c
@@ -988,7 +974,6 @@ EXTERN_C __attribute__((__noreturn__)) void ApeLoader(long di, long *sp,
     prog = (char *)sp[3];
     argc = sp[3] = sp[0] - 3;
     argv = (char **)((sp += 3) + 1);
-    may_path_search = 0;
   } else if (argc < 2) {
     ShowUsage(os, 2, 1);
   } else {
@@ -1029,8 +1014,7 @@ EXTERN_C __attribute__((__noreturn__)) void ApeLoader(long di, long *sp,
   }
 
   /* resolve path of executable and read its first page */
-  if (!(exe = Commandv(&M->ps, os, prog, GetEnv(envp, "PATH"),
+  if (!(exe = Commandv(&M->ps, os, prog, GetEnv(envp, "PATH")))) {
-                       may_path_search))) {
     Pexit(os, prog, 0, "not found (maybe chmod +x or ./ needed)");
   } else if ((fd = Open(exe, O_RDONLY, 0, os)) < 0) {
     Pexit(os, exe, fd, "open");