Re: [PATCH v2 3/3] coredump: hand a pidfd to the usermode coredump helper

[Benjamin Drung <benjamin.drung@canonical.com>]

On Fri, 2025-04-25 at 13:57 +0200, Christian Brauner wrote:
> On Fri, Apr 25, 2025 at 01:31:56PM +0200, Benjamin Drung wrote:
> > 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@gmail.com>
> > > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > > ---
> > >  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
> 
> I have no clue what -F%F is and whether that leading -F is something
> specific to Apport but the specifier is %F not -F%F. For example:
> 
>         > cat /proc/sys/kernel/core_pattern
>         |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %F
> 
> And note that this requires the pipe logic to be used, aka "|" needs to
> be specified. Without it this doesn't make sense.

Apport takes short option parameters. They match the kernel template
specifiers. The failing pattern:

$ cat /proc/sys/kernel/core_pattern 
|/usr/share/apport/apport -p%p -s%s -c%c -d%d -P%P -u%u -g%g -F%F -- %E

Once I drop %F Apport is called without issues:

$ cat /proc/sys/kernel/core_pattern 
|/usr/share/apport/apport -p%p -s%s -c%c -d%d -P%P -u%u -g%g -- %E

-- 
Benjamin Drung
Debian & Ubuntu Developer