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References: <20221124-kexec-noalloc-v1-0-d78361e99aec@chromium.org> <20221128180003.49747650@rotkaeppchen>
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From:   Ricardo Ribalda <ribalda@chromium.org>
Date:   Mon, 28 Nov 2022 18:07:06 +0100
Message-ID: <CANiDSCuwE8_bVErcKV0UcvaPjMx4vbgXzhw2dY69_x2GcB0VxQ@mail.gmail.com>
Subject: Re: [PATCH] kexec: Enable runtime allocation of crash_image
To:     Philipp Rudo <prudo@redhat.com>
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
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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.

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,
>


-- 
Ricardo Ribalda