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Wed, 30 Oct 2024 16:38:20 -0400 (EDT) From: Tycho Andersen To: Alexander Viro , Christian Brauner , Jan Kara , Eric Biederman , Kees Cook , Shuah Khan Cc: =?utf-8?q?Zbigniew_J=C4=99drzejewski-Szmek?= , Aleksa Sarai , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Tycho Andersen , Tycho Andersen Subject: [PATCH 1/2] exec: fix up /proc/pid/comm in the execveat(AT_EMPTY_PATH) case Date: Wed, 30 Oct 2024 14:37:31 -0600 Message-Id: <20241030203732.248767-1-tycho@tycho.pizza> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Tycho Andersen Zbigniew mentioned at Linux Plumber's that systemd is interested in switching to execveat() for service execution, but can't, because the contents of /proc/pid/comm are the file descriptor which was used, instead of the path to the binary. This makes the output of tools like top and ps useless, especially in a world where most fds are opened CLOEXEC so the number is truly meaningless. Change exec path to fix up /proc/pid/comm in the case where we have allocated one of these synthetic paths in bprm_init(). This way the actual exec machinery is unchanged, but cosmetically the comm looks reasonable to admins investigating things. Signed-off-by: Tycho Andersen Suggested-by: Zbigniew Jędrzejewski-Szmek CC: Aleksa Sarai Link: https://github.com/uapi-group/kernel-features#set-comm-field-before-exec --- v2: * drop the flag, everyone :) * change the rendered value to f_path.dentry->d_name.name instead of argv[0], Eric v3: * fix up subject line, Eric v4: * switch to no flag, always rewrite approach, with some cleanup suggested by Kees --- fs/exec.c | 36 +++++++++++++++++++++++++++++++++++- include/linux/binfmts.h | 1 + 2 files changed, 36 insertions(+), 1 deletion(-) base-commit: c1e939a21eb111a6d6067b38e8e04b8809b64c4e diff --git a/fs/exec.c b/fs/exec.c index 6c53920795c2..3b559f598c74 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1347,7 +1347,16 @@ int begin_new_exec(struct linux_binprm * bprm) set_dumpable(current->mm, SUID_DUMP_USER); perf_event_exec(); - __set_task_comm(me, kbasename(bprm->filename), true); + + /* + * If argv0 was set, alloc_bprm() made up a path that will + * probably not be useful to admins running ps or similar. + * Let's fix it up to be something reasonable. + */ + if (bprm->argv0) + __set_task_comm(me, kbasename(bprm->argv0), true); + else + __set_task_comm(me, kbasename(bprm->filename), true); /* An exec changes our domain. We are no longer part of the thread group */ @@ -1497,9 +1506,28 @@ static void free_bprm(struct linux_binprm *bprm) if (bprm->interp != bprm->filename) kfree(bprm->interp); kfree(bprm->fdpath); + kfree(bprm->argv0); kfree(bprm); } +static int bprm_add_fixup_comm(struct linux_binprm *bprm, + struct user_arg_ptr argv) +{ + const char __user *p = get_user_arg_ptr(argv, 0); + + /* + * If p == NULL, let's just fall back to fdpath. + */ + if (!p) + return 0; + + bprm->argv0 = strndup_user(p, MAX_ARG_STRLEN); + if (bprm->argv0) + return 0; + + return -EFAULT; +} + static struct linux_binprm *alloc_bprm(int fd, struct filename *filename, int flags) { struct linux_binprm *bprm; @@ -1906,6 +1934,12 @@ static int do_execveat_common(int fd, struct filename *filename, goto out_ret; } + if (unlikely(bprm->fdpath)) { + retval = bprm_add_fixup_comm(bprm, argv); + if (retval != 0) + goto out_free; + } + retval = count(argv, MAX_ARG_STRINGS); if (retval == 0) pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n", diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h index e6c00e860951..bab5121a746b 100644 --- a/include/linux/binfmts.h +++ b/include/linux/binfmts.h @@ -55,6 +55,7 @@ struct linux_binprm { of the time same as filename, but could be different for binfmt_{misc,script} */ const char *fdpath; 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Wed, 30 Oct 2024 16:38:24 -0400 (EDT) From: Tycho Andersen To: Alexander Viro , Christian Brauner , Jan Kara , Eric Biederman , Kees Cook , Shuah Khan Cc: =?utf-8?q?Zbigniew_J=C4=99drzejewski-Szmek?= , Aleksa Sarai , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Tycho Andersen , Tycho Andersen Subject: [PATCH 2/2] selftests/exec: add a test for execveat()'s comm Date: Wed, 30 Oct 2024 14:37:32 -0600 Message-Id: <20241030203732.248767-2-tycho@tycho.pizza> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20241030203732.248767-1-tycho@tycho.pizza> References: <20241030203732.248767-1-tycho@tycho.pizza> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Tycho Andersen In the previous patch we've defined a couple behaviors: 1. execveat(fd, AT_EMPTY_PATH, {"foo"}, ...) should render argv[0] as /proc/pid/comm 2. execveat(fd, AT_EMPTY_PATH, {NULL}, ...) should keep the old behavior of rendering the fd as /proc/pid/comm and just to be sure keeps working with symlinks, which was a concern in [1], I've added a test for that as well. The test itself is a bit ugly, because the existing check_execveat_fail() helpers use a hardcoded envp and argv, and we want to "pass" things via the environment to test various argument values, but it seemed cleaner than passing one in everywhere in all the existing tests. Output looks like: ok 51 Check success of execveat(6, 'home/tycho/packages/...yyyyyyyyyyyyyyyyyyyy', 0)... # Check execveat(AT_EMPTY_PATH)'s comm is sentinel ok 52 Check success of execveat(9, '', 4096)... # Check execveat(AT_EMPTY_PATH)'s comm is sentinel ok 53 Check success of execveat(11, '', 4096)... # Check execveat(AT_EMPTY_PATH)'s comm is 9 [ 25.579272] process 'execveat' launched '/dev/fd/9' with NULL argv: empty string added ok 54 Check success of execveat(9, '', 4096)... [1]: https://lore.kernel.org/all/20240925.152228-private.conflict.frozen.trios-TdUGhuI5Sb4v@cyphar.com/ Signed-off-by: Tycho Andersen --- v4: fix up commit message, use ksft_perror() vs perror(), Shuah --- tools/testing/selftests/exec/execveat.c | 77 ++++++++++++++++++++++++- 1 file changed, 74 insertions(+), 3 deletions(-) diff --git a/tools/testing/selftests/exec/execveat.c b/tools/testing/selftests/exec/execveat.c index 071e03532cba..3a05f8cbd815 100644 --- a/tools/testing/selftests/exec/execveat.c +++ b/tools/testing/selftests/exec/execveat.c @@ -23,9 +23,11 @@ #include "../kselftest.h" -#define TESTS_EXPECTED 51 +#define TESTS_EXPECTED 54 #define TEST_NAME_LEN (PATH_MAX * 4) +#define CHECK_COMM "CHECK_COMM" + static char longpath[2 * PATH_MAX] = ""; static char *envp[] = { "IN_TEST=yes", NULL, NULL }; static char *argv[] = { "execveat", "99", NULL }; @@ -237,12 +239,36 @@ static int check_execveat_pathmax(int root_dfd, const char *src, int is_script) return fail; } +static int check_execveat_comm(int fd, char *argv0, char *expected) +{ + char buf[128], *old_env, *old_argv0; + int ret; + + snprintf(buf, sizeof(buf), CHECK_COMM "=%s", expected); + + old_env = envp[1]; + envp[1] = buf; + + old_argv0 = argv[0]; + argv[0] = argv0; + + ksft_print_msg("Check execveat(AT_EMPTY_PATH)'s comm is %s\n", + expected); + ret = check_execveat_invoked_rc(fd, "", AT_EMPTY_PATH, 0, 0); + + envp[1] = old_env; + argv[0] = old_argv0; + + return ret; +} + static int run_tests(void) { int fail = 0; char *fullname = realpath("execveat", NULL); char *fullname_script = realpath("script", NULL); char *fullname_symlink = concat(fullname, ".symlink"); + char fd_buf[10]; int subdir_dfd = open_or_die("subdir", O_DIRECTORY|O_RDONLY); int subdir_dfd_ephemeral = open_or_die("subdir.ephemeral", O_DIRECTORY|O_RDONLY); @@ -389,6 +415,15 @@ static int run_tests(void) fail += check_execveat_pathmax(root_dfd, "execveat", 0); fail += check_execveat_pathmax(root_dfd, "script", 1); + + /* /proc/pid/comm gives argv[0] by default */ + fail += check_execveat_comm(fd, "sentinel", "sentinel"); + /* /proc/pid/comm gives argv[0] when invoked via link */ + fail += check_execveat_comm(fd_symlink, "sentinel", "sentinel"); + /* /proc/pid/comm gives fdno if NULL is passed */ + snprintf(fd_buf, sizeof(fd_buf), "%d", fd); + fail += check_execveat_comm(fd, NULL, fd_buf); + return fail; } @@ -415,9 +450,13 @@ int main(int argc, char **argv) int ii; int rc; const char *verbose = getenv("VERBOSE"); + const char *check_comm = getenv(CHECK_COMM); - if (argc >= 2) { - /* If we are invoked with an argument, don't run tests. */ + if (argc >= 2 || check_comm) { + /* + * If we are invoked with an argument, or no arguments but a + * command to check, don't run tests. + */ const char *in_test = getenv("IN_TEST"); if (verbose) { @@ -426,6 +465,38 @@ int main(int argc, char **argv) ksft_print_msg("\t[%d]='%s\n'", ii, argv[ii]); } + /* If the tests wanted us to check the command, do so. */ + if (check_comm) { + /* TASK_COMM_LEN == 16 */ + char buf[32]; + int fd, ret; + + fd = open("/proc/self/comm", O_RDONLY); + if (fd < 0) { + ksft_perror("open() comm failed"); + exit(1); + } + + ret = read(fd, buf, sizeof(buf)); + if (ret < 0) { + ksft_perror("read() comm failed"); + close(fd); + exit(1); + } + close(fd); + + // trim off the \n + buf[ret-1] = 0; + + if (strcmp(buf, check_comm)) { + ksft_print_msg("bad comm, got: %s expected: %s", + buf, check_comm); + exit(1); + } + + exit(0); + } + /* Check expected environment transferred. */ if (!in_test || strcmp(in_test, "yes") != 0) { ksft_print_msg("no IN_TEST=yes in env\n");