Fix crash on cd in subshell with PWD unset (re: 5ee290c)

Reproducer:

$ ksh -c 'unset PWD; (cd /); :'
Memory fault

The shell crashes because b_cd() is testing the value of the PWD
variable without checking if there is one.

src/cmd/ksh93/sh/path.c: path_pwd():
- Never return an unfreeable pointer to e_dot; always return a
  freeable pointer. This fixes another corner-case crashing bug.
- Make sure the PWD variable gets assigned a value if it doesn't
  have one, even if it's the "." fallback. However, if the PWD is
  inaccessible but we did inherit a $PWD value that starts with a
  /, then use the existing $PWD value as this will help the shell
  fail gracefully.

src/cmd/ksh93/bltins/cd_pwd.c:
- b_cd(): When checking if the PWD is valid, use the sh.pwd copy
  instead of the PWD variable. This fixes the crash above.
- b_cd(): Since path_pwd() now always returns a freeable value,
  free sh.pwd unconditionally before setting the new value.
- b_pwd(): Not only check that path_pwd() returns a value starting
  with a slash, but also verify it with test_inode() and error out
  if it's wrong. This makes the 'pwd' command useful for checking
  that the PWD is currently accessible.

src/cmd/ksh93/data/msg.c:
- Change e_pwd error message for accuracy and clarity.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-01-17:
+
+- Fixed a crash, introduced on 2021-01-19, that occurred when using 'cd' in
+  a subshell with the PWD variable unset.
+
 2022-01-16:
 
 - Backported minor additions to the 'read' built-in command from ksh 93v-:

src/cmd/ksh93/bltins/cd_pwd.c

@@ -100,8 +100,6 @@ int	b_cd(int argc, char *argv[],Shbltin_t *context)
 	oldpwd = path_pwd();
 	opwdnod = sh_scoped(OLDPWDNOD);
 	pwdnod = sh_scoped(PWDNOD);
-	if(oldpwd == e_dot && pwdnod->nvalue.cp)
-		oldpwd = (char*)pwdnod->nvalue.cp;  /* if path_pwd() failed to get the pwd, use $PWD */
 	if(sh.subshell)
 	{
 		/* clone $OLDPWD and $PWD into the subshell's scope */
@@ -126,7 +124,7 @@ int	b_cd(int argc, char *argv[],Shbltin_t *context)
 	if(sh.subshell && !sh.subshare)
 	{
 #if _lib_fchdir
-		if(!test_inode(nv_getval(pwdnod),e_dot))
+		if(!test_inode(sh.pwd,e_dot))
 #endif
 			sh_subfork();
 	}
@@ -228,6 +226,7 @@ success:
 	if(*dp && (*dp!='.'||dp[1]) && strchr(dir,'/'))
 		sfputr(sfstdout,dir,'\n');
 	nv_putval(opwdnod,oldpwd,NV_RDONLY);
+	free((void*)sh.pwd);
 	if(*dir == '/')
 	{
 		size_t len = strlen(dir);
@@ -236,16 +235,12 @@ success:
 			dir[len] = 0;
 		nv_putval(pwdnod,dir,NV_RDONLY);
 		nv_onattr(pwdnod,NV_EXPORT);
-		if(sh.pwd)
-			free((void*)sh.pwd);
-		sh.pwd = sh_strdup(pwdnod->nvalue.cp);
+		sh.pwd = sh_strdup(dir);
 	}
 	else
 	{
 		/* pathcanon() failed to canonicalize the directory, which happens when 'cd' is invoked from a
 		   nonexistent PWD with a relative path as the argument. Reinitialize $PWD as it will be wrong. */
-		if(sh.pwd)
-			free((void*)sh.pwd);
 		sh.pwd = NIL(const char*);
 		path_pwd();
 		if(*sh.pwd != '/')
@@ -291,7 +286,7 @@ int	b_pwd(int argc, char *argv[],Shbltin_t *context)
 		errormsg(SH_DICT,ERROR_usage(2),"%s",optusage((char*)0));
 		UNREACHABLE();
 	}
-	if(*(cp = path_pwd()) != '/')
+	if(*(cp = path_pwd()) != '/' || !test_inode(cp,e_dot))
 	{
 		errormsg(SH_DICT,ERROR_system(1), e_pwd);
 		UNREACHABLE();

src/cmd/ksh93/data/msg.c

@@ -78,7 +78,7 @@ const char e_badpattern[]	= "%s: invalid shell pattern";
 const char e_noread[]		= "%s: pattern seek requires read access";
 const char e_logout[]		= "Use 'exit' to terminate this shell";
 const char e_exec[]		= "%s: cannot execute";
-const char e_pwd[]		= "cannot access parent directories";
+const char e_pwd[]		= "cannot determine present working directory";
 const char e_found[]		= "%s: not found";
 #ifdef ENAMETOOLONG
 const char e_toolong[]		= "%s: file name too long";

src/cmd/ksh93/include/version.h

@@ -21,7 +21,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-beta.2"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2022-02-16"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2022-02-17"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2022 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh/path.c

@@ -186,7 +186,11 @@ char *path_pwd(void)
 	Namval_t *pwdnod;
 	/* Don't bother if PWD already set */
 	if(sh.pwd)
-		return((char*)sh.pwd);
+	{
+		if(*sh.pwd=='/')
+			return((char*)sh.pwd);
+		free((void*)sh.pwd);
+	}
 	/* First see if PWD variable is correct */
 	pwdnod = sh_scoped(PWDNOD);
 	cp = nv_getval(pwdnod);
@@ -198,22 +202,29 @@ char *path_pwd(void)
 		cp = nv_getval(sh_scoped(HOME));
 		if(!(cp && *cp=='/' && test_inode(cp,e_dot)))
 		{
-			/* Get physical PWD (no symlinks) using getcwd(3), fall back to "." */
+			/* Get physical PWD (no symlinks) using getcwd(3) */
 			cp = sh_getcwd();
-			if(!cp)
-				return((char*)e_dot);
-			tofree++;
+			if(cp)
+				tofree++;
 		}
 		/* Store in PWD variable */
-		if(sh.subshell)
-			pwdnod = sh_assignok(pwdnod,1);
-		nv_putval(pwdnod,cp,NV_RDONLY);
+		if(cp)
+		{
+			if(sh.subshell)
+				pwdnod = sh_assignok(pwdnod,1);
+			nv_putval(pwdnod,cp,NV_RDONLY);
+		}
 		if(tofree)
-			free(cp);
+			free((void*)cp);
 	}
 	nv_onattr(pwdnod,NV_EXPORT);
+	/* Neither obtained the pwd nor can fall back to sane-ish $PWD: fall back to "." */
+	if(!cp)
+		cp = nv_getval(pwdnod);
+	if(!cp || *cp!='/')
+		nv_putval(pwdnod,cp=(char*)e_dot,NV_RDONLY);
 	/* Set shell PWD */
-	sh.pwd = sh_strdup(pwdnod->nvalue.cp);
+	sh.pwd = sh_strdup(cp);
 	return((char*)sh.pwd);
 }
 

src/cmd/ksh93/tests/path.sh

@@ -931,5 +931,19 @@ then	(
 	) 2>/dev/null || err_exit "'source' in POSIX mode does not find ksh function"
 fi
 
+# ======
+# Crash after unsetting PWD
+(unset PWD; (cd /); :) &	# the : avoids optimizing out the subshell
+wait "$!" 2>/dev/null
+((!(e = $?))) || err_exit "shell crashes on 'cd' in subshell exit with unset PWD" \
+	"(got status $e$( ((e>128)) && print -n /SIG && kill -l "$e"))"
+mkdir "$tmp/testdir"
+cd "$tmp/testdir"
+"$SHELL" -c 'cd /; rmdir "$1"' x "$tmp/testdir"
+(unset PWD; exec "$SHELL" -c '(cd /); :') &
+wait "$!" 2>/dev/null
+((!(e = $?))) || err_exit 'shell crashes on failure obtain the PWD on init' \
+	"(got status $e$( ((e>128)) && print -n /SIG && kill -l "$e"))"
+
 # ======
 exit $((Errors<125?Errors:125))