ash: command -v CMD must skip (go to next path) when CMD exists, but is not executable

Upstream commit:

    Date: Fri, 5 Apr 2024 17:55:46 +0800
    exec: Check executable bit when searching path

    Andrej Shadura <andrew.shadura@collabora.co.uk> wrote:
    ...
    > https://bugs.debian.org/874264
    > -------- Forwarded Message --------
    > Subject: dash: 'command -v' mistakenly returns a shell script whose
    > executable is not set
    > Date: Mon, 04 Sep 2017 10:45:48 -0400
    > From: Norman Ramsey <nr@cs.tufts.edu>
    > To: Debian Bug Tracking System <submit@bugs.debian.org>
    ...
    > I tracked a build bug in s-nail to a problem with dash.  Symptom:
    > building s-nail tries to run /home/nr/bin/clang, a script whose
    > executable bit is not set.  We tracked the problem to the result of
    > running `command -v clang` with /bin/sh:
    ...
    This is inherited from NetBSD.  There is even a commented-out
    block of code that tried to fix this.

    Anyway, we now have faccessat so we can simply use it.

function                                             old     new   delta
test_exec                                              -     125    +125
find_command                                         911     918      +7
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/0 up/down: 132/0)             Total: 132 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/ash.c

@@ -491,6 +491,11 @@ struct globals_misc {
 	char **trap_ptr;        /* used only by "trap hack" */
 
 	/* Rarely referenced stuff */
+
+	/* Cached supplementary group array (for testing executable'ity of files) */
+	int ngroups;
+	gid_t *group_array;
+
 #if ENABLE_ASH_RANDOM_SUPPORT
 	random_t random_gen;
 #endif
@@ -523,6 +528,8 @@ extern struct globals_misc *BB_GLOBAL_CONST ash_ptr_to_globals_misc;
 #define may_have_traps    (G_misc.may_have_traps   )
 #define trap        (G_misc.trap       )
 #define trap_ptr    (G_misc.trap_ptr   )
+#define ngroups     (G_misc.ngroups    )
+#define group_array (G_misc.group_array)
 #define random_gen  (G_misc.random_gen )
 #define backgndpid  (G_misc.backgndpid )
 #define INIT_G_misc() do { \
@@ -10171,6 +10178,7 @@ static int FAST_FUNC printfcmd(int argc, char **argv) { return printf_main(argc,
 #endif
 #if ENABLE_ASH_TEST || BASH_TEST2
 static int FAST_FUNC testcmd(int argc, char **argv)   { return test_main(argc, argv); }
+// TODO: pass &ngroups and &group_array addresses to test_main to use cached supplementary groups
 #endif
 #if ENABLE_ASH_SLEEP
 static int FAST_FUNC sleepcmd(int argc, char **argv)  { return sleep_main(argc, argv); }
@@ -13782,6 +13790,43 @@ readcmdfile(char *name)
 
 /* ============ find_command inplementation */
 
+static int test_exec(/*const char *fullname,*/ struct stat *statb)
+{
+	/*
+	 * TODO: use faccessat(AT_FDCWD, fullname, X_OK, AT_EACCESS)
+	 * instead: executability may depend on ACLs, capabilities
+	 * and who knows what else, not just mode bits.
+	 * (faccessat2 syscall was added to Linux in May 14 2020)
+	 */
+	mode_t stmode;
+	uid_t euid;
+	enum { ANY_IX = S_IXUSR | S_IXGRP | S_IXOTH };
+
+	/* Do we already know with no extra syscalls? */
+	if (!S_ISREG(statb->st_mode))
+		return 0; /* not a regular file */
+	if ((statb->st_mode & ANY_IX) == 0)
+		return 0; /* no one can execute */
+	if ((statb->st_mode & ANY_IX) == ANY_IX)
+		return 1; /* anyone can execute */
+
+	/* Executability depends on our euid/egid/supplementary groups */
+	stmode = S_IXOTH;
+	euid = geteuid();
+//TODO: cache euid?
+	if (euid == 0)
+		/* for root user, any X bit is good enough */
+		stmode = ANY_IX;
+	else if (statb->st_uid == euid)
+		stmode = S_IXUSR;
+	else if (statb->st_gid == getegid())
+		stmode = S_IXGRP;
+	else if (is_in_supplementary_groups(&ngroups, &group_array, statb->st_gid))
+		stmode = S_IXGRP;
+
+	return statb->st_mode & stmode;
+}
+
 /*
  * Resolve a command name.  If you change this routine, you may have to
  * change the shellexec routine as well.
@@ -13808,9 +13853,12 @@ find_command(char *name, struct cmdentry *entry, int act, const char *path)
 				if (errno == EINTR)
 					continue;
 #endif
+ absfail:
 				entry->cmdtype = CMDUNKNOWN;
 				return;
 			}
+			if (!test_exec(/*name,*/ &statb))
+				goto absfail;
 		}
 		entry->cmdtype = CMDNORMAL;
 		return;
@@ -13923,9 +13971,6 @@ find_command(char *name, struct cmdentry *entry, int act, const char *path)
 				e = errno;
 			goto loop;
 		}
-		e = EACCES;     /* if we fail, this will be the error */
-		if (!S_ISREG(statb.st_mode))
-			continue;
 		if (lpathopt) {          /* this is a %func directory */
 			stalloc(len);
 			/* NB: stalloc will return space pointed by fullname
@@ -13938,6 +13983,9 @@ find_command(char *name, struct cmdentry *entry, int act, const char *path)
 			stunalloc(fullname);
 			goto success;
 		}
+		e = EACCES;     /* if we fail, this will be the error */
+		if (!test_exec(/*fullname,*/ &statb))
+			continue;
 		TRACE(("searchexec \"%s\" returns \"%s\"\n", name, fullname));
 		if (!updatetbl) {
 			entry->cmdtype = CMDNORMAL;