libnss: read /proc/self/cmdline

src/libnss.zig

@@ -87,9 +87,13 @@ fn init() void {
                 );
             }
         }
-        //if (os.argv.len == 0) break :blk false;
+
-        //break :blk mem.eql(u8, mem.sliceTo(os.argv[0], 0), "id");
+        // argv does not exist because
-        break :blk false; // https://github.com/ziglang/zig/issues/4524#issuecomment-1184748756
+        // https://github.com/ziglang/zig/issues/4524#issuecomment-1184748756
+        // so reading /proc/self/cmdline because
+        const fd = os.openZ("/proc/self/cmdline", os.O.RDONLY, 0) catch break :blk false;
+        defer os.close(fd);
+        break :blk isId(fd);
     };
 
     const fname = if (getenv(ENV_DB)) |env|
@@ -540,6 +544,25 @@ fn getDBErrno(errnop: *c_int) ?*const DB {
     }
 }
 
+// isId tells if this command is "id". Reads the cmdline
+// from the given fd. Returns false on any error.
+fn isId(fd: os.fd_t) bool {
+    var cmdline: [256]u8 = undefined;
+    const nb = os.read(fd, cmdline[0..]) catch return false;
+    const arg0 = mem.sliceTo(cmdline[0..nb], 0);
+
+    if (arg0.len < 2)
+        return false;
+
+    if (!mem.eql(u8, arg0[arg0.len - 2 .. arg0.len], "id"))
+        return false;
+
+    if (arg0.len == 2)
+        return true;
+
+    return arg0[arg0.len - 3] == '/';
+}
+
 const testing = std.testing;
 
 test "libnss getpwuid_r and getpwnam_r" {
@@ -675,3 +698,27 @@ test "libnss initgroups_dyn" {
     try testing.expectEqual(@as(u32, 9999), groups[1]);
     try testing.expectEqual(@as(u32, 100000), groups[2]);
 }
+
+test "libnss isId" {
+    const examples = [_]struct {
+        cmdline: []const u8,
+        want: bool,
+    }{
+        .{ .want = true, .cmdline = "id\x00foo" },
+        .{ .want = true, .cmdline = "/usr/bin/id\x00foo" },
+        .{ .want = true, .cmdline = "id" },
+        .{ .want = true, .cmdline = "/usr/bin/id" },
+        .{ .want = false, .cmdline = "" },
+        .{ .want = false, .cmdline = "d\x00" },
+        .{ .want = false, .cmdline = "/fakeid" },
+    };
+
+    for (examples) |tt| {
+        const fd = try os.memfd_create("test_libnss_isId", 0);
+        defer os.close(fd);
+        _ = try os.write(fd, tt.cmdline);
+        try os.lseek_SET(fd, 0);
+        const got = isId(fd);
+        try testing.expectEqual(tt.want, got);
+    }
+}