Let chmod work on folders bound into the guest

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "localdesktop"
-version = "3.1.0"
+version = "3.1.1"
 edition = "2021"
 build = "build.rs"
 default-run = "build_apk"

patches/build-proot-android/README.md

@@ -25,4 +25,12 @@ One of them is in `src/syscall/seccomp.c`: the seccomp filter traps only the
 instead of every `ioctl`, so GPU drivers in the guest (Turnip on KGSL) don't
 stop at the tracer on each command submission.
 
+Another is in `src/extension/fake_id0/chmod.c`: `chmod` on a file outside the
+guest rootfs is still dropped, except on the app's own files (a directory bound
+into the guest, such as `/home`), so a script or binary written there can be
+made executable.
+
+The shipped `libproot.so` is built with `./build.sh` (NDK r26d). On macOS, put a
+GNU `install` (or a wrapper that handles `install -D`) first in `PATH`.
+
 See https://github.com/green-green-avk/proot for more info.

patches/build-proot-android/build/proot/src/extension/fake_id0/chmod.c

@@ -1,4 +1,6 @@
 #include <errno.h>
+#include <sys/stat.h>
+#include <unistd.h>
 #include <linux/limits.h>
 
 #include "syscall/sysnum.h"
@@ -28,8 +30,13 @@ int handle_chmod_enter_end(Tracee *tracee, Reg path_sysarg, Reg mode_sysarg,
 		status = read_sysarg_path(tracee, path, path_sysarg, CURRENT);
 	if(status < 0)
 		return status;
-	// If the file exists outside the guestfs, drop the syscall.
+	// If the file exists outside the guestfs, drop the syscall, unless it is
+	// the app's own (a directory bound into the guest, such as /home): then
+	// let the real chmod through, or an executable written there never runs.
 	else if(status == 1) {
+		struct stat host;
+		if(stat(path, &host) == 0 && host.st_uid == getuid())
+			return 0;
 		set_sysnum(tracee, PR_getuid);
 		return 0;
 	}