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Date:   Tue, 29 Nov 2022 16:12:44 +0100
From:   Philipp Rudo <prudo@redhat.com>
To:     Ricardo Ribalda <ribalda@chromium.org>
Cc:     Eric Biederman <ebiederm@xmission.com>,
        Baoquan He <bhe@redhat.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Ross Zwisler <zwisler@kernel.org>, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kexec: Enable runtime allocation of crash_image
Message-ID: <20221129161244.7ecf59e5@rotkaeppchen>
In-Reply-To: <CANiDSCuwE8_bVErcKV0UcvaPjMx4vbgXzhw2dY69_x2GcB0VxQ@mail.gmail.com>
References: <20221124-kexec-noalloc-v1-0-d78361e99aec@chromium.org>
        <20221128180003.49747650@rotkaeppchen>
        <CANiDSCuwE8_bVErcKV0UcvaPjMx4vbgXzhw2dY69_x2GcB0VxQ@mail.gmail.com>
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Ricardo,

On Mon, 28 Nov 2022 18:07:06 +0100
Ricardo Ribalda <ribalda@chromium.org> wrote:

> Hi Philipp
> 
> 
> Thanks for your review.
> 
> 
> On Mon, 28 Nov 2022 at 18:00, Philipp Rudo <prudo@redhat.com> wrote:
> >
> > Hi Ricardo,
> >
> > On Thu, 24 Nov 2022 23:23:36 +0100
> > Ricardo Ribalda <ribalda@chromium.org> wrote:
> >  
> > > Usually crash_image is defined statically via the crashkernel parameter
> > > or DT.
> > >
> > > But if the crash kernel is not used, or is smaller than then
> > > area pre-allocated that memory is wasted.
> > >
> > > Also, if the crash kernel was not defined at bootime, there is no way to
> > > use the crash kernel.
> > >
> > > Enable runtime allocation of the crash_image if the crash_image is not
> > > defined statically. Following the same memory allocation/validation path
> > > that for the reboot kexec kernel.
> > >
> > > Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>  
> >
> > I don't think this patch will work as intended. For one you omit
> > setting the image->type to KEXEC_TYPE_CRASH. But when you grep for that
> > type you will find that there is a lot of special handling done for it.
> > I don't believe that this can simply be skipped without causing
> > problems.
> >
> > Furthermore I think you have missed one important detail. The memory
> > reserved for the crash kernel is not just a buffer for the image but
> > the memory it runs in! For that it has to be a continuous piece of
> > physical memory with usually some additional arch specific limitations.
> > When allocated dynamically all those limitations need to be considered.
> > But a standard kexec doesn't care about those limitations as it doesn't
> > care about the os running before itself. It can simply overwrite the
> > memory when booting. But if the crash kernel does the same it will
> > corrupt the dump it is supposed to generate.  
> 
> Right now, I do not intend to use it to fetch a kdump, I am using it
> as the image that will run when the system crashes.

the crash_image is currently all about creating a dump. If you want to
change that you need to discuss the new behavior in the commit message!
Please update the commit message.

Thanks
Philipp

> 
> It seems to work fine on the two devices that I am using for tests.
> 
> >
> > Thanks
> > Philipp
> >  
> > > ---
> > > kexec: Enable runtime allocation of crash_image
> > >
> > > To: Eric Biederman <ebiederm@xmission.com>
> > > Cc: kexec@lists.infradead.org
> > > Cc: linux-kernel@vger.kernel.org
> > > Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
> > > Cc: linux-kernel@vger.kernel.org
> > > Cc: Ross Zwisler <zwisler@kernel.org>
> > > Cc: Philipp Rudo <prudo@redhat.com>
> > > Cc: Baoquan He <bhe@redhat.com>
> > > ---
> > >  include/linux/kexec.h | 1 +
> > >  kernel/kexec.c        | 9 +++++----
> > >  kernel/kexec_core.c   | 5 +++++
> > >  kernel/kexec_file.c   | 7 ++++---
> > >  4 files changed, 15 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> > > index 41a686996aaa..98ca9a32bc8e 100644
> > > --- a/include/linux/kexec.h
> > > +++ b/include/linux/kexec.h
> > > @@ -427,6 +427,7 @@ extern int kexec_load_disabled;
> > >  extern bool kexec_in_progress;
> > >
> > >  int crash_shrink_memory(unsigned long new_size);
> > > +bool __crash_memory_valid(void);
> > >  ssize_t crash_get_memory_size(void);
> > >
> > >  #ifndef arch_kexec_protect_crashkres
> > > diff --git a/kernel/kexec.c b/kernel/kexec.c
> > > index cb8e6e6f983c..b5c17db25e88 100644
> > > --- a/kernel/kexec.c
> > > +++ b/kernel/kexec.c
> > > @@ -28,7 +28,7 @@ static int kimage_alloc_init(struct kimage **rimage, unsigned long entry,
> > >       struct kimage *image;
> > >       bool kexec_on_panic = flags & KEXEC_ON_CRASH;
> > >
> > > -     if (kexec_on_panic) {
> > > +     if (kexec_on_panic && __crash_memory_valid()) {
> > >               /* Verify we have a valid entry point */
> > >               if ((entry < phys_to_boot_phys(crashk_res.start)) ||
> > >                   (entry > phys_to_boot_phys(crashk_res.end)))
> > > @@ -44,7 +44,7 @@ static int kimage_alloc_init(struct kimage **rimage, unsigned long entry,
> > >       image->nr_segments = nr_segments;
> > >       memcpy(image->segment, segments, nr_segments * sizeof(*segments));
> > >
> > > -     if (kexec_on_panic) {
> > > +     if (kexec_on_panic && __crash_memory_valid()) {
> > >               /* Enable special crash kernel control page alloc policy. */
> > >               image->control_page = crashk_res.start;
> > >               image->type = KEXEC_TYPE_CRASH;
> > > @@ -101,7 +101,7 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
> > >
> > >       if (flags & KEXEC_ON_CRASH) {
> > >               dest_image = &kexec_crash_image;
> > > -             if (kexec_crash_image)
> > > +             if (kexec_crash_image && __crash_memory_valid())
> > >                       arch_kexec_unprotect_crashkres();
> > >       } else {
> > >               dest_image = &kexec_image;
> > > @@ -157,7 +157,8 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
> > >       image = xchg(dest_image, image);
> > >
> > >  out:
> > > -     if ((flags & KEXEC_ON_CRASH) && kexec_crash_image)
> > > +     if ((flags & KEXEC_ON_CRASH) && kexec_crash_image &&
> > > +         __crash_memory_valid())
> > >               arch_kexec_protect_crashkres();
> > >
> > >       kimage_free(image);
> > > diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> > > index ca2743f9c634..77083c9760fb 100644
> > > --- a/kernel/kexec_core.c
> > > +++ b/kernel/kexec_core.c
> > > @@ -1004,6 +1004,11 @@ void crash_kexec(struct pt_regs *regs)
> > >       }
> > >  }
> > >
> > > +bool __crash_memory_valid(void)
> > > +{
> > > +     return crashk_res.end != crashk_res.start;
> > > +}
> > > +
> > >  ssize_t crash_get_memory_size(void)
> > >  {
> > >       ssize_t size = 0;
> > > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > > index 45637511e0de..0671f4f370ff 100644
> > > --- a/kernel/kexec_file.c
> > > +++ b/kernel/kexec_file.c
> > > @@ -280,7 +280,7 @@ kimage_file_alloc_init(struct kimage **rimage, int kernel_fd,
> > >
> > >       image->file_mode = 1;
> > >
> > > -     if (kexec_on_panic) {
> > > +     if (kexec_on_panic && __crash_memory_valid()) {
> > >               /* Enable special crash kernel control page alloc policy. */
> > >               image->control_page = crashk_res.start;
> > >               image->type = KEXEC_TYPE_CRASH;
> > > @@ -345,7 +345,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
> > >       dest_image = &kexec_image;
> > >       if (flags & KEXEC_FILE_ON_CRASH) {
> > >               dest_image = &kexec_crash_image;
> > > -             if (kexec_crash_image)
> > > +             if (kexec_crash_image && __crash_memory_valid())
> > >                       arch_kexec_unprotect_crashkres();
> > >       }
> > >
> > > @@ -408,7 +408,8 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
> > >  exchange:
> > >       image = xchg(dest_image, image);
> > >  out:
> > > -     if ((flags & KEXEC_FILE_ON_CRASH) && kexec_crash_image)
> > > +     if ((flags & KEXEC_FILE_ON_CRASH) && kexec_crash_image &&
> > > +         __crash_memory_valid())
> > >               arch_kexec_protect_crashkres();
> > >
> > >       kexec_unlock();
> > >
> > > ---
> > > base-commit: 4312098baf37ee17a8350725e6e0d0e8590252d4
> > > change-id: 20221124-kexec-noalloc-3cab3cbe000f
> > >
> > > Best regards,  
> >  
> 
>