Add a wait mode that uses pidfd_open()

.github/workflows/build-and-test.yml

@@ -3,10 +3,10 @@ name: Build and test with CMake
 on:
   push:
     branches:
-    - master
+    - main
   pull_request:
     branches:
-    - master
+    - main
 
 jobs:
   build:

src/CMakeLists.txt

@@ -6,7 +6,7 @@ include(CheckSymbolExists)
 check_symbol_exists(__GLIBC__ features.h _GNU_SOURCE)
 configure_file(config.h.in config.h)
 
-add_executable(pwait pwait.c ptrace.c netlink.c poll.c capabilities.c)
+add_executable(pwait pwait.c ptrace.c netlink.c poll.c pidfd.c capabilities.c)
 target_link_libraries(pwait cap)
 
 install(TARGETS pwait RUNTIME DESTINATION bin)

src/pidfd.c

@@ -0,0 +1,71 @@
+/* Implements waiting for a process using the `pidfd_open` syscall
+ *
+ * This implementation is similar to the one in
+ * [the man page for `pidfd_open`](https://man7.org/linux/man-pages/man2/pidfd_open.2.html)
+ * but that's effectively forced by the task; there isn't a lot of variety in
+ * how you can write this.
+ */
+
+#define _GNU_SOURCE
+
+#include "pwait.h"
+#include <assert.h>
+#include <errno.h>
+#include <poll.h>
+#include <sysexits.h>
+#include <unistd.h>
+#include <sys/syscall.h>
+
+
+/**
+ * Open a process file descriptor.
+ *
+ * This is a thin wrapper around the pidfd_open() syscall.
+ *
+ * @return The file descriptor opened, or -1 if opening failed.
+ */
+static int pidfd_open(pid_t pid) {
+    long result = syscall(SYS_pidfd_open, pid, 0);
+    int pidfd = (int)result;
+    assert((long)pidfd == result);
+    return pidfd;
+}
+
+
+int wait_using_pidfd(pid_t pid) {
+    int fd = pidfd_open(pid);
+    if (fd < 0) {
+        // ESRCH indicates that no process with that ID was found
+        return errno == ESRCH ? -1 : EX_OSERR;
+    }
+
+    struct pollfd pfd = {
+        .fd     = fd,
+        .events = POLLIN,
+    };
+
+    int pfd_status;
+    while ((pfd_status = poll(&pfd, 1, -1)) >= 0) {
+        if (pfd_status < 0) {
+            if (errno == EINTR) {
+                // poll() was interrupted by a signal, so we can just keep going
+                continue;
+            }
+            else {
+                // Some more serious error occurred
+                return EX_OSERR;
+            }
+        }
+        else if (pfd_status == 0) {
+            // This shouldn't happen because we set timeout to -1, i.e. infinite
+            return -1;
+        }
+        else {
+            assert(pfd_status == 1);
+            if (pfd.revents & POLLIN) {
+                break;
+            }
+        }
+    }
+    return 0;
+}

src/pwait.c

@@ -45,7 +45,7 @@ static void help(const char* name) {
     printf("  -d, --delay=SECONDS  set the polling frequency when --method=poll\n");
     printf("  -h, --help           print this help message and exit\n");
     printf("  -m, --method=METHOD  use METHOD to wait for the process\n");
-    printf("                       METHOD is one of 'netlink' (default), 'ptrace', or 'poll'\n");
+    printf("                       METHOD is one of 'netlink' (default), 'ptrace', 'poll', or 'pidfd'\n");
     printf("  -v, --verbose        print diagnostic output to stderr\n");
 #else
     printf("  -h    print this help message and exit\n");
@@ -101,6 +101,9 @@ int main(const int argc, char* const* argv) {
                 else if (strncmp(optarg, "poll", 5) == 0) {
                     wait_function = wait_using_polling;
                 }
+                else if (strncmp(optarg, "pidfd", 6) == 0) {
+                    wait_function = wait_using_pidfd;
+                }
                 else {
                     wait_function = NULL;
                 }

src/pwait.h

@@ -29,4 +29,5 @@ int acquire_capabilities(size_t n, const cap_value_t* capabilities_to_acquire);
 int wait_using_ptrace(pid_t pid);
 int wait_using_netlink(pid_t pid);
 int wait_using_polling(pid_t pid);
+int wait_using_pidfd(pid_t pid);
 void set_delay(unsigned int delay);

test/CMakeLists.txt

@@ -24,4 +24,10 @@ add_test(
     NAME test-pwait-poll
     COMMAND "${CMAKE_CURRENT_LIST_DIR}/test_pwait.sh"
 )
-set_tests_properties(test-pwait-poll PROPERTIES ENVIRONMENT "PWAIT=$<TARGET_FILE:pwait>;PWAIT_METHOD=poll")
+set_tests_properties(test-pwait-poll PROPERTIES ENVIRONMENT "PWAIT=$<TARGET_FILE:pwait>;PWAIT_METHOD=poll;PWAIT_SKIP_EXIT_CODE_TESTS=1")
+
+add_test(
+    NAME test-pwait-pidfd
+    COMMAND "${CMAKE_CURRENT_LIST_DIR}/test_pwait.sh"
+)
+set_tests_properties(test-pwait-pidfd PROPERTIES ENVIRONMENT "PWAIT=$<TARGET_FILE:pwait>;PWAIT_METHOD=pidfd;PWAIT_SKIP_EXIT_CODE_TESTS=1")

test/test_pwait.sh

@@ -127,9 +127,11 @@ test_pwait_and_target_exit_times() {
 
 
 run_all_tests() {
-    test_pwait_exit_code 0
-    test_pwait_exit_code 1
-    test_pwait_exit_code 128
+    if [[ -z "${PWAIT_SKIP_EXIT_CODE_TESTS:-}" ]]; then
+        test_pwait_exit_code 0
+        test_pwait_exit_code 1
+        test_pwait_exit_code 128
+    fi
     test_target_does_not_exist_after_pwait_exit
     test_pwait_and_target_exit_times 0.1s
     test_pwait_and_target_exit_times 1s
@@ -138,23 +140,17 @@ run_all_tests() {
 }
 
 
-run_poll_tests() {
-    pwait_options=("--method=poll")
-    test_target_does_not_exist_after_pwait_exit
+run_poll_delay_tests() {
     for delay in 1 2 5; do
         pwait_options=("--method=poll" "--delay=$delay")
         test_pwait_and_target_exit_times 10s "${delay}"
     done
 }
 
 
-pwait_options=()
-case "${PWAIT_METHOD:-}" in
-    poll)
-        run_poll_tests
-    ;;
-    *)
-        pwait_options=("--method=${PWAIT_METHOD}")
-        run_all_tests
-    ;;
-esac
+pwait_options=("--method=${PWAIT_METHOD}")
+run_all_tests
+
+if [[ "${PWAIT_METHOD:-}" == "poll" ]]; then
+    run_poll_delay_tests
+fi