Hi, On Mon, 2025-04-14 at 15:55 +0200, Christian Brauner wrote: > 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@xxxxxxxxx> > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> > --- > fs/coredump.c | 59 ++++++++++++++++++++++++++++++++++++++++++++---- > include/linux/coredump.h | 1 + > 2 files changed, 56 insertions(+), 4 deletions(-) > > diff --git a/fs/coredump.c b/fs/coredump.c > index 9da592aa8f16..403be0ff780e 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 +348,27 @@ 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': { > + /* > + * Installing a pidfd only makes sense if > + * we actually spawn a usermode helper. > + */ > + if (!ispipe) > + break; > + > + /* > + * 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. > + */ > + cprm->pid = task_tgid(current); > + err = cn_printf(cn, "%d", COREDUMP_PIDFD_NUMBER); > + break; > + } > default: > break; > } > I tried this change with Apport: I took the Ubuntu mainline kernel build https://kernel.ubuntu.com/mainline/daily/2025-04-24/ (that refers to mainline commit e54f9b0410347c49b7ffdd495578811e70d7a407) and applied these three patches on top. Then I modified Apport to take the additional `-F%F` and tested that on Ubuntu 25.04 (plucky). The result is the coredump failed as long as there was `-F%F` on /proc/sys/kernel/core_pattern: ``` coredump: 7392(divide-by-zero): |/usr/share/apport/apport pipe failed ``` Did I do something wrong? Do I miss additional patches? -- Benjamin Drung Debian & Ubuntu Developer
