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Message-Id: <20250414-work-coredump-v1-3-6caebc807ff4@kernel.org>
Date: Mon, 14 Apr 2025 11:53:38 +0200
From: Christian Brauner <brauner@...nel.org>
To: linux-fsdevel@...r.kernel.org
Cc: Oleg Nesterov <oleg@...hat.com>,
Luca Boccassi <luca.boccassi@...il.com>,
Lennart Poettering <lennart@...ttering.net>,
Daan De Meyer <daan.j.demeyer@...il.com>, Mike Yuan <me@...dnzj.com>,
Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>,
linux-kernel@...r.kernel.org, Christian Brauner <brauner@...nel.org>
Subject: [PATCH 3/3] coredump: hand a pidfd to the usermode coredump helper
Give userspace a way to instruct the kernel to install a pidfd into the
usermode helper process. This makes coredump handling a lot more
reliable for userspace. In parallel with this commit we already have
systemd adding support for this in [1].
We create a pidfs file for the coredumping process when we process the
corename pattern. When the usermode helper process is forked we then
install the pidfs file as file descriptor three into the usermode
helpers file descriptor table so it's available to the exec'd program.
Since usermode helpers are either children of the system_unbound_wq
workqueue or kthreadd we know that the file descriptor table is empty
and can thus always use three as the file descriptor number.
Note, that we'll install a pidfd for the thread-group leader even if a
subthread is calling do_coredump(). We know that task linkage hasn't
been removed due to delay_group_leader() and even if this @current isn't
the actual thread-group leader we know that the thread-group leader
cannot be reaped until @current has exited.
Link: https://github.com/systemd/systemd/pull/37125 [1]
Tested-by: Luca Boccassi <luca.boccassi@...il.com>
Signed-off-by: Christian Brauner <brauner@...nel.org>
---
fs/coredump.c | 85 +++++++++++++++++++++++++++++++++++++++++++++---
include/linux/coredump.h | 1 +
2 files changed, 82 insertions(+), 4 deletions(-)
diff --git a/fs/coredump.c b/fs/coredump.c
index 2ed5e6956a09..ae0c1c5efe9a 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -43,6 +43,9 @@
#include <linux/timekeeping.h>
#include <linux/sysctl.h>
#include <linux/elf.h>
+#include <linux/pidfs.h>
+#include <uapi/linux/pidfd.h>
+#include <linux/vfsdebug.h>
#include <linux/uaccess.h>
#include <asm/mmu_context.h>
@@ -60,6 +63,12 @@ static void free_vma_snapshot(struct coredump_params *cprm);
#define CORE_FILE_NOTE_SIZE_DEFAULT (4*1024*1024)
/* Define a reasonable max cap */
#define CORE_FILE_NOTE_SIZE_MAX (16*1024*1024)
+/*
+ * File descriptor number for the pidfd for the thread-group leader of
+ * the coredumping task installed into the usermode helper's file
+ * descriptor table.
+ */
+#define COREDUMP_PIDFD_NUMBER 3
static int core_uses_pid;
static unsigned int core_pipe_limit;
@@ -339,6 +348,58 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
case 'C':
err = cn_printf(cn, "%d", cprm->cpu);
break;
+ /* pidfd number */
+ case 'F': {
+ struct file *pidfs_file __free(fput) = NULL;
+
+ /*
+ * Install a pidfd only makes sense if
+ * we actually spawn a usermode helper.
+ */
+ if (!ispipe)
+ break;
+
+ /*
+ * We already created a pidfs_file but the user
+ * specified F multiple times. Just print the
+ * number multiple times.
+ */
+ if (!cprm->pidfs_file) {
+ /*
+ * Create a pidfs file for the
+ * coredumping thread that we can
+ * install into the usermode helper's
+ * file descriptor table later.
+ *
+ * Note that we'll install a pidfd for
+ * the thread-group leader. We know that
+ * task linkage hasn't been removed yet
+ * and even if this @current isn't the
+ * actual thread-group leader we know
+ * that the thread-group leader cannot
+ * be reaped until @current has exited.
+ */
+ pidfs_file = pidfs_alloc_file(task_tgid(current), 0);
+ if (IS_ERR(pidfs_file))
+ return PTR_ERR(pidfs_file);
+ }
+
+ /*
+ * Usermode helpers are childen of
+ * either system_unbound_wq or of
+ * kthreadd. So we know that we're
+ * starting off with a clean file
+ * descriptor table. Thus, we should
+ * always be able to use file descriptor
+ * number 3.
+ */
+ err = cn_printf(cn, "%d", COREDUMP_PIDFD_NUMBER);
+ if (err)
+ return err;
+
+ cprm->pidfs_file = no_free_ptr(pidfs_file);
+ break;
+ }
default:
break;
}
@@ -493,7 +554,7 @@ static void wait_for_dump_helpers(struct file *file)
}
/*
- * umh_pipe_setup
+ * umh_coredump_setup
* helper function to customize the process used
* to collect the core in userspace. Specifically
* it sets up a pipe and installs it as fd 0 (stdin)
@@ -503,12 +564,26 @@ static void wait_for_dump_helpers(struct file *file)
* is a special value that we use to trap recursive
* core dumps
*/
-static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
+static int umh_coredump_setup(struct subprocess_info *info, struct cred *new)
{
struct file *files[2];
struct coredump_params *cp = (struct coredump_params *)info->data;
+ struct file *pidfs_file = cp->pidfs_file;
int err;
+ if (pidfs_file) {
+ /* We must start from a pristine file descriptor table. */
+ VFS_WARN_ON_ONCE((pidfs_file = fget_raw(COREDUMP_PIDFD_NUMBER)) != NULL);
+
+ /*
+ * Takes it's own reference on success and do_coredump()
+ * will put the additional reference.
+ */
+ err = replace_fd(COREDUMP_PIDFD_NUMBER, pidfs_file, 0);
+ if (err < 0)
+ return err;
+ }
+
err = create_pipe_files(files, 0);
if (err)
return err;
@@ -598,7 +673,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
}
if (cprm.limit == 1) {
- /* See umh_pipe_setup() which sets RLIMIT_CORE = 1.
+ /* See umh_coredump_setup() which sets RLIMIT_CORE = 1.
*
* Normally core limits are irrelevant to pipes, since
* we're not writing to the file system, but we use
@@ -637,7 +712,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
retval = -ENOMEM;
sub_info = call_usermodehelper_setup(helper_argv[0],
helper_argv, NULL, GFP_KERNEL,
- umh_pipe_setup, NULL, &cprm);
+ umh_coredump_setup, NULL, &cprm);
if (sub_info)
retval = call_usermodehelper_exec(sub_info,
UMH_WAIT_EXEC);
@@ -784,6 +859,8 @@ void do_coredump(const kernel_siginfo_t *siginfo)
if (ispipe)
atomic_dec(&core_dump_count);
fail_unlock:
+ if (cprm.pidfs_file)
+ fput(cprm.pidfs_file);
kfree(argv);
kfree(cn.corename);
coredump_finish(core_dumped);
diff --git a/include/linux/coredump.h b/include/linux/coredump.h
index 77e6e195d1d6..af2ed4b68dcf 100644
--- a/include/linux/coredump.h
+++ b/include/linux/coredump.h
@@ -28,6 +28,7 @@ struct coredump_params {
int vma_count;
size_t vma_data_size;
struct core_vma_metadata *vma_meta;
+ struct file *pidfs_file;
};
extern unsigned int core_file_note_size_limit;
--
2.47.2
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