0.18.3 --- a directory handle names a directory and does not read one (#42)

Co-authored-by: speak-agent <248744407+speak-agent@users.noreply.github.com>

Author: Sunrisepeak

README.md

@@ -8,7 +8,7 @@ for Linux, written on the kernel's own system-call interface.
 openkal = "0.17.0"
 
 [target.'cfg(os = "linux")'.dependencies]
-openkal-linux = "0.18.2"
+openkal-linux = "0.18.3"
 ```
 
 ## Why it does not use a C library

mcpp.toml

@@ -1,7 +1,7 @@
 [package]
 namespace   = "mcpplibs"
 name        = "openkal-linux"
-version     = "0.18.2"
+version     = "0.18.3"
 description = "The reference implementation of openkal for Linux, written on the kernel's own system-call interface so that it can be placed beneath a C library as well as above one."
 license     = "Apache-2.0"
 

src/fs.cpp

@@ -82,6 +82,25 @@ bool granted(preopen* t, kal_uintptr* n) {
     return true;
 }
 
+// A DIRECTORY HANDLE NAMES A DIRECTORY; IT DOES NOT READ ONE (0.18.3).
+//
+// Every `kal_dir' is a base: names are resolved beneath it (`*at'), a program
+// is started in it (`fchdir'), and it is described (`fstat', `fstatfs'). None
+// of these reads the directory, and `O_PATH' is the open that asks for no
+// access but naming. Reading happens only where entries are listed, and an
+// enumeration already opens a descriptor of its own through the handle, with
+// read access, at that moment.
+//
+// Until 0.18.2 the handles were opened for reading, and a directory the
+// program may search but not read could not be a base at all. Measured on
+// Android 14 under `run-as' (SELinux domain `runas_app', mcpp#857): the
+// preopen of `/' failed with EACCES, so every name resolved beneath the root,
+// and every program started by absolute name, failed before anything was
+// started --- reported as "not found", because the base it would have been
+// found in was never opened. The same program, its directory and its exec
+// path all worked when each step was called directly.
+constexpr okl_long dir_handle_flags = okl::o_path | okl::o_directory | okl::o_cloexec;
+
 preopen* table(kal_uintptr* count) {
     static preopen t[kMaxGrants];
     static kal_uintptr n = 2;
@@ -102,11 +121,9 @@ preopen* table(kal_uintptr* count) {
         else { g_cwd[0] = '.'; g_cwd[1] = '\0'; cwd_len = 1; }
 
         const okl_long fd0 = okl::sys(okl::nr_openat, okl::at_fdcwd,
-                                      reinterpret_cast<okl_long>("."),
-                                      okl::o_rdonly | okl::o_directory | okl::o_cloexec, 0);
+                                      reinterpret_cast<okl_long>("."), dir_handle_flags, 0);
         const okl_long fd1 = okl::sys(okl::nr_openat, okl::at_fdcwd,
-                                      reinterpret_cast<okl_long>("/"),
-                                      okl::o_rdonly | okl::o_directory | okl::o_cloexec, 0);
+                                      reinterpret_cast<okl_long>("/"), dir_handle_flags, 0);
         t[0] = { g_cwd, cwd_len, okl::failed(fd0) ? 0u : okl::pack(static_cast<int>(fd0)) };
         t[1] = { "/",   1,       okl::failed(fd1) ? 0u : okl::pack(static_cast<int>(fd1)) };
     }
@@ -398,7 +415,7 @@ int kal_fs_open_dir(kal_dir base, const char* name, kal_uintptr len, kal_dir* ou
     if (b < 0 || out == nullptr || !okl::acceptable(name, len)) return kal_err_invalid;
     okl::terminated t(name, len); if (!t.ok) return kal_err_invalid;
     const okl_long fd = okl::sys(okl::nr_openat, b, reinterpret_cast<okl_long>(t.buf),
-                                 okl::o_rdonly | okl::o_directory | okl::o_cloexec, 0);
+                                 dir_handle_flags, 0);
     if (okl::failed(fd)) return okl::translate(fd);
     *out = kal_dir{ okl::pack(static_cast<int>(fd)) };
     return kal_ok;

src/process.cpp

@@ -344,7 +344,11 @@ inline okl_long handle_pid(kal_process h) { return static_cast<okl_long>(h.h & 0
 // the duplicate still allocates nothing. It costs the property `execveat'
 // bought, that the name is resolved through the descriptor, and is paid only
 // where `execveat' is unusable: from the first start under a translator this
-// implementation recognises, and after the first ENOSYS anywhere else.
+// implementation recognises, and, elsewhere, by each start that meets ENOSYS,
+// which falls back in the started image itself. That fallback is learned by
+// no other start: the image is a copy made by `clone' without shared memory,
+// so nothing it learns reaches this program, and the price is one refused
+// system call per start.
 //
 // Ported from the copy mcppls carried (vendor/openkal-linux 0.15.1), which set
 // the flag itself under PRoot; recognising the translator here removes that.

tests/search_only_base.cpp

@@ -0,0 +1,89 @@
+// A directory the program may search but not read is still a base (0.18.3).
+//
+// A `kal_dir' names a directory; resolving beneath it, starting a program in
+// it and describing it read nothing, so none of them needs the right to read
+// it. Only listing does. Until 0.18.2 every handle was opened for reading, and
+// on Android 14 under `run-as' (SELinux `runas_app') the root preopen itself
+// failed with EACCES: every program started by absolute name was reported as
+// not found (mcpp#857).
+//
+// Here the same situation is made with a mode instead of a policy: a directory
+// of mode 0111. A privileged user reads it regardless, so the one assertion
+// that it cannot be listed is made only for an unprivileged one.
+#include <cstdio>
+#include <cstring>
+#include <sys/stat.h>
+#include <unistd.h>
+import openkal.fs;
+import openkal.stream;
+import openkal.types;
+import openkal.process;
+
+namespace {
+int failures = 0;
+void check(bool held, const char* what) {
+    if (!held) { std::printf("FAIL: %s\n", what); ++failures; }
+}
+
+kal_dir root() {
+    for (kal_uintptr i = 0; i < kal::fs::preopen_count(); ++i) {
+        kal_dir d{}; char name[512]; kal_uintptr len = 0;
+        if (kal_fs_preopen(i, &d, name, sizeof name, &len) == kal_ok && len == 1 && name[0] == '/')
+            return d;
+    }
+    return kal_dir{};
+}
+}
+
+int main() {
+    const auto work = kal::fs::working();
+    check(kal_fs_mkdir(work, "okl_search_only", 15) == kal_ok, "a directory is created");
+    kal_dir d{};
+    check(kal_fs_open_dir(work, "okl_search_only", 15, &d) == kal_ok, "it opens");
+    kal_file f{};
+    check(kal::fs::open_file(d, "inside", 6, kal::fs::open::write | kal::fs::open::create, &f) == kal_ok,
+          "a file is created in it");
+    kal_stream_write(kal_fs_stream(f), "x", 1);
+    kal_fs_close_file(f);
+    kal_fs_close_dir(d);
+
+    check(::chmod("okl_search_only", 0111) == 0, "the directory loses its read right");
+
+    kal_dir s{};
+    check(kal_fs_open_dir(work, "okl_search_only", 15, &s) == kal_ok,
+          "a searchable directory without the read right opens as a base");
+    kal_file g{};
+    check(kal::fs::open_file(s, "inside", 6, kal::fs::open::read, &g) == kal_ok,
+          "a name beneath it resolves");
+    char c = 0;
+    check(kal_stream_read(kal_fs_stream(g), &c, 1) == 1 && c == 'x', "the file beneath it reads");
+    kal_fs_close_file(g);
+
+    // A program is started IN it: the base it is resolved under is the root
+    // preopen, and its working directory is the search-only directory.
+    const char* argv[] = { "true" };
+    const kal_uintptr lens[] = { 4 };
+    const kal_spawn how{ root(), s, nullptr, nullptr, 0, 0 };
+    kal_process p{};
+    const int rc = kal_process_spawn(&how, "bin/true", 8, argv, lens, 1, nullptr, nullptr, 0, nullptr, &p);
+    check(rc == kal_ok, "a program starts in a search-only directory");
+    if (rc == kal_ok) {
+        int status = -1, terminated = -1;
+        kal_process_wait(p, &status, &terminated);
+        kal_process_close(p);
+        check(terminated == 0 && status == 0, "the program ran to completion");
+    }
+
+    if (::geteuid() != 0) {
+        kal_uintptr iter = 0;
+        check(kal_fs_list_begin(s, &iter) == kal_err_permission,
+              "listing it is what the missing read right refuses");
+    }
+    kal_fs_close_dir(s);
+    ::chmod("okl_search_only", 0755);
+    ::unlink("okl_search_only/inside");
+    ::rmdir("okl_search_only");
+
+    if (failures == 0) std::printf("search_only_base: ok\n");
+    return failures == 0 ? 0 : 1;
+}