Added PATH lookup for program argument in sink.

Fortune for sink's current commit: Half blessing − 半吉

Makefile

@@ -1,4 +1,5 @@
 # `sink` - sinks all input to /dev/null
+# Usage: `sink [<program> [<args...>]]`
 #
 # Targets:
 # `sink` (default): stripped `release` target. output: `sink`

README.md

@@ -1,6 +1,16 @@
 # `sink` - sinks all input into `/dev/null`
 
-Re-routes `stdin`, `stdout` (and optionally, `stderr`) to `/dev/null`.
+Re-routes `stdin`, `stdout` (and optionally, `stderr`) of a program to `/dev/null`.
+
+## Usage
+Command usage is in the form of: `./sink [<program name> [<arguments...>]]`.
+
+Example:
+```shell
+$ sink /usr/bin/cat sink.c # Identical to `>>/dev/null 2>&1 cat sink.c`
+```
+
+When invoked with `<program name>`: `execve()`s into the program (if it exists or is found in `$PATH`) with `<arguments....>` and a passed-through `envp` (see below in *Compiler flags* on changing this behaviour.)
 
 ## Building
 
@@ -10,6 +20,8 @@ To build for debugging, run `make debug`.
 
 ### Compiler flags
 `-DREPLACE_STDERR` - Add to `CFLAGS` when building a target to re-route the program's `stderr` stream to the sink as well. By default, it is closed after the other streams have been successfully rewritten.
+`-DNO_SEARCH_PATH` - Add to `CFLAGS` when building to prevent the program looking up its argument in the `PATH` environment variable.
+`-DNO_ENV` - Add to `CFLAGS` when building to prevent `envp` passthrough to the `execve()`'d program.
 
 ### Installation
 The program must have been built before installation, and installation and uninstallation must be done as root.

sink.c

@@ -1,6 +1,10 @@
 #include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <limits.h>
+#include <errno.h>
 
 #define r_stdin 0
 #define r_stdout 1
@@ -22,25 +26,112 @@ static inline int dupall(int from)
 #ifdef REPLACE_STDERR
 	if(UNLIKELY(dup2(from, r_stderr) < 0)) {
 		perror("failed to dup2() stderr to sink");
-#else
+/*#else
 	if(UNLIKELY(close(r_stderr) != 0)) {
 		perror("failed to close stderr");
-		
-#endif
+		*/
 		return 3;
 	}
+#endif
 	return 0;
 }
 
-int main(void)
+inline static int err_not_found(int er) {
+	return er == ENOENT;
+}
+
+static inline int path_exists(const char fname[restrict static 1])
+{
+	return access(fname, F_OK | X_OK) != -1;
+}
+
+static int path_lookup(size_t sz; const char name[restrict static 1], char fullpath[restrict sz], size_t sz, const char envpath[restrict static 1])
+{
+	char* paths = strdup(envpath);
+	char* _tmpfree = paths;
+	int found = 0;
+	const char* item;
+
+	while((item = strsep(&paths, ":")) != NULL) {
+		if(UNLIKELY( ((size_t)snprintf(fullpath, sz, "%s/%s", item, name)) >= sz )) {
+#if defined(DEBUG) && !defined(REPLACE_STDERR)
+			fprintf(stderr, "Warning: normalised path item '%s' truncated (from %s/%s). Full path was longer than %lu bytes\n", fullpath, item, name, sz);
+#endif
+			errno = ENAMETOOLONG;
+			continue;
+		}
+		if(path_exists(fullpath)) {
+			found = 1;
+			break;
+		}
+	}
+
+	free(_tmpfree);
+	return found;
+}
+
+int main(int argc, char** argv, char** envp)
 {
+	(void)argc;
+#ifdef NO_ENV
+#define GET_ENV ((char*[]){})
+	(void)envp;
+#else
+#define GET_ENV envp
+#endif
+
+#define $execve(path) execve((path), argv+1, GET_ENV)
+
 	int null = open("/dev/null", O_RDWR);
 	if(UNLIKELY(null<0)) {
 		perror("failed to open /dev/null");
 		return 1;
 	}
 	register int rc = dupall(null);
-	if(LIKELY(rc)) close(null);
-	return rc;
+	if(LIKELY(!rc)) close(null);
+	else return rc;
+
+#ifdef REPLACE_STDERR
+#define perror(v) ((void)(v)) 
+#define eprintf(...) ((void)(__VA_ARGS__))
+#else
+#define eprintf(...) fprintf(stderr, __VA_ARGS__)
+#endif
+
+
+	if(argv[1]) {
+#if NO_SEARCH_PATH
+		if(UNLIKELY($execve(argv[1]) < 0)) {
+			perror("execve() failed");
+			return errno;
+		}
+#else
+#define $try_execve(path) do { if(UNLIKELY($execve((path)) < 0))  { perror("execve() failed"); return errno; } else __builtin_unreachable(); } while(0)
+		const char* path = NULL;
+		int err = 0;
+		if(LIKELY(($execve(argv[1]) < 0)
+			&& (err_not_found(err = errno))
+			&& (path = getenv("PATH")))) {
+			// Failed to exec raw pathname (not found), lookup in $PATH
+			char fullpath[PATH_MAX+1];
+			errno = 0;
+			if(path_lookup(argv[1], fullpath, PATH_MAX, path)) $try_execve(fullpath);
+			else {
+				eprintf("Error: failed to find %s in PATH\n", argv[1]);
+				return errno ?: -1;
+			}
+		} else {
+			if(UNLIKELY(!err)) err = errno;
+
+			if(err_not_found(err)) {
+				perror(path 
+					? "execve() failed. Program not found in PATH"
+					: "execve() failed. No PATH set to look through");
+			} else	perror("execve() failed");
+			return err ?: -1;
+		}
+#endif
+	}
+	return 0;
 }