From db67752672877ea0f5360699cb34ad31065ef39e Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Sun, 13 Jun 2021 15:53:06 -0400
Subject: spawn: New function for resolving executables in the PATH

This fixes the BFS_SPAWN_USEPATH to use the parent's environment, not
the new child's environment, to resolve the executable.
---
 spawn.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++++++++------------
 spawn.h | 13 +++++++++
 2 files changed, 91 insertions(+), 17 deletions(-)

diff --git a/spawn.c b/spawn.c
index 82beca5..f83663a 100644
--- a/spawn.c
+++ b/spawn.c
@@ -17,7 +17,9 @@
 #include "spawn.h"
 #include "util.h"
 #include <errno.h>
+#include <fcntl.h>
 #include <stdlib.h>
+#include <string.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
@@ -139,21 +141,9 @@ int bfs_spawn_addsetrlimit(struct bfs_spawn *ctx, int resource, const struct rli
 	}
 }
 
-/** Facade for execvpe() which is non-standard. */
-static int bfs_execvpe(const char *exe, char **argv, char **envp) {
-#if __GLIBC__ || __linux__ || __NetBSD__ || __OpenBSD__
-	return execvpe(exe, argv, envp);
-#else
-	extern char **environ;
-	environ = envp;
-	return execvp(exe, argv);
-#endif
-}
-
 /** Actually exec() the new process. */
 static void bfs_spawn_exec(const char *exe, const struct bfs_spawn *ctx, char **argv, char **envp, int pipefd[2]) {
 	int error;
-	enum bfs_spawn_flags flags = ctx ? ctx->flags : 0;
 	const struct bfs_spawn_action *actions = ctx ? ctx->actions : NULL;
 
 	close(pipefd[0]);
@@ -199,11 +189,7 @@ static void bfs_spawn_exec(const char *exe, const struct bfs_spawn *ctx, char **
 		}
 	}
 
-	if (flags & BFS_SPAWN_USEPATH) {
-		bfs_execvpe(exe, argv, envp);
-	} else {
-		execve(exe, argv, envp);
-	}
+	execve(exe, argv, envp);
 
 fail:
 	error = errno;
@@ -222,6 +208,15 @@ pid_t bfs_spawn(const char *exe, const struct bfs_spawn *ctx, char **argv, char
 		envp = environ;
 	}
 
+	enum bfs_spawn_flags flags = ctx ? ctx->flags : 0;
+	char *resolved = NULL;
+	if (flags & BFS_SPAWN_USEPATH) {
+		exe = resolved = bfs_spawn_resolve(exe);
+		if (!resolved) {
+			return -1;
+		}
+	}
+
 	// Use a pipe to report errors from the child
 	int pipefd[2];
 	if (pipe_cloexec(pipefd) != 0) {
@@ -244,6 +239,7 @@ pid_t bfs_spawn(const char *exe, const struct bfs_spawn *ctx, char **argv, char
 
 	// Parent
 	close(pipefd[1]);
+	free(resolved);
 
 	ssize_t nbytes = xread(pipefd[0], &error, sizeof(error));
 	close(pipefd[0]);
@@ -256,3 +252,68 @@ pid_t bfs_spawn(const char *exe, const struct bfs_spawn *ctx, char **argv, char
 
 	return pid;
 }
+
+char *bfs_spawn_resolve(const char *exe) {
+	if (strchr(exe, '/')) {
+		return strdup(exe);
+	}
+
+	const char *path = getenv("PATH");
+
+	char *confpath = NULL;
+	if (!path) {
+		path = confpath = xconfstr(_CS_PATH);
+		if (!path) {
+			return NULL;
+		}
+	}
+
+	size_t cap = 0;
+	char *ret = NULL;
+	while (true) {
+		const char *end = strchr(path, ':');
+		size_t len = end ? (size_t)(end - path) : strlen(path);
+
+		// POSIX 8.3: "A zero-length prefix is a legacy feature that
+		// indicates the current working directory."
+		if (len == 0) {
+			path = ".";
+			len = 1;
+		}
+
+		size_t total = len + 1 + strlen(exe) + 1;
+		if (cap < total) {
+			char *grown = realloc(ret, total);
+			if (!grown) {
+				goto fail;
+			}
+			ret = grown;
+			cap = total;
+		}
+
+		memcpy(ret, path, len);
+		if (ret[len - 1] != '/') {
+			ret[len++] = '/';
+		}
+		strcpy(ret + len, exe);
+
+		if (xfaccessat(AT_FDCWD, ret, X_OK) == 0) {
+			break;
+		}
+
+		if (!end) {
+			errno = ENOENT;
+			goto fail;
+		}
+
+		path = end + 1;
+	}
+
+	free(confpath);
+	return ret;
+
+fail:
+	free(confpath);
+	free(ret);
+	return NULL;
+}
diff --git a/spawn.h b/spawn.h
index 16815bb..7bae89c 100644
--- a/spawn.h
+++ b/spawn.h
@@ -107,4 +107,17 @@ int bfs_spawn_addsetrlimit(struct bfs_spawn *ctx, int resource, const struct rli
  */
 pid_t bfs_spawn(const char *exe, const struct bfs_spawn *ctx, char **argv, char **envp);
 
+/**
+ * Look up an executable in the current PATH, as BFS_SPAWN_USEPATH or execvp()
+ * would do.
+ *
+ * @param exe
+ *         The name of the binary to execute.  Bare names without a '/' will be
+ *         searched on the provided PATH.
+ * @return
+ *         The full path to the executable, which should be free()'d, or NULL on
+ *         failure.
+ */
+char *bfs_spawn_resolve(const char *exe);
+
 #endif // BFS_SPAWN_H
-- 
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