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References: <20230321-kexec_clang16-v4-0-1340518f98e9@chromium.org>
 <20230321-kexec_clang16-v4-1-1340518f98e9@chromium.org> <ZCU+63H7GzPlL6QJ@kernel.org>
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From:   Ricardo Ribalda <ribalda@chromium.org>
Date:   Thu, 30 Mar 2023 11:47:14 +0200
Message-ID: <CANiDSCsbmmkfRGC-GvhX-RSZGU0CT7ZqLo6Z=RmccoM7zbPcsA@mail.gmail.com>
Subject: Re: [PATCH v4 1/2] kexec: Support purgatories with .text.hot sections
To:     Simon Horman <horms@kernel.org>
Cc:     Eric Biederman <ebiederm@xmission.com>,
        linux-kernel@vger.kernel.org, Baoquan He <bhe@redhat.com>,
        stable@vger.kernel.org, Ross Zwisler <zwisler@google.com>,
        Philipp Rudo <prudo@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>, kexec@lists.infradead.org
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Hi Simon

Thanks for your review!

On Thu, 30 Mar 2023 at 09:49, Simon Horman <horms@kernel.org> wrote:
>
> On Mon, Mar 27, 2023 at 05:06:53PM +0200, Ricardo Ribalda wrote:
> > Clang16 links the purgatory text in two sections:
> >
> >   [ 1] .text             PROGBITS         0000000000000000  00000040
> >        00000000000011a1  0000000000000000  AX       0     0     16
> >   [ 2] .rela.text        RELA             0000000000000000  00003498
> >        0000000000000648  0000000000000018   I      24     1     8
> >   ...
> >   [17] .text.hot.        PROGBITS         0000000000000000  00003220
> >        000000000000020b  0000000000000000  AX       0     0     1
> >   [18] .rela.text.hot.   RELA             0000000000000000  00004428
> >        0000000000000078  0000000000000018   I      24    17     8
> >
> > And both of them have their range [sh_addr ... sh_addr+sh_size] on the
> > area pointed by `e_entry`.
> >
> > This causes that image->start is calculated twice, once for .text and
> > another time for .text.hot. The second calculation leaves image->start
> > in a random location.
> >
> > Because of this, the system crashes inmediatly after:
>
> s/inmediatly/immediately/
>
> >
> > kexec_core: Starting new kernel
> >
> > Cc: stable@vger.kernel.org
>
> Maybe a fixes tag is warranted here.
>
> > Reviewed-by: Ross Zwisler <zwisler@google.com>
> > Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
> > ---
> >  kernel/kexec_file.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > index f1a0e4e3fb5c..25a37d8f113a 100644
> > --- a/kernel/kexec_file.c
> > +++ b/kernel/kexec_file.c
> > @@ -901,10 +901,21 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
> >               }
> >
> >               offset = ALIGN(offset, align);
> > +
> > +             /*
> > +              * Check if the segment contains the entry point, if so,
> > +              * calculate the value of image->start based on it.
> > +              * If the compiler has produced more than one .text sections
>
> nit: s/sections/section/
>
> > +              * (Eg: .text.hot), they are generally after the main .text
>
> If this is the general case, then are there cases where this doesn't hold?

When looking at this issue, I have only seen .text.hot after .text.
But I cannot warantee that future versions of llvm or gcc decide to
swap the order.

I am going to add a WARN whenever there are two overlapping .text
sections so the user has the chance to update their linker script.

>
> > +              * section, and they shall not be used to calculate
> > +              * image->start. So do not re-calculate image->start if it
> > +              * is not set to the initial value.
> > +              */
> >               if (sechdrs[i].sh_flags & SHF_EXECINSTR &&
> >                   pi->ehdr->e_entry >= sechdrs[i].sh_addr &&
> >                   pi->ehdr->e_entry < (sechdrs[i].sh_addr
> > -                                      + sechdrs[i].sh_size)) {
> > +                                      + sechdrs[i].sh_size) &&
> > +                 kbuf->image->start == pi->ehdr->e_entry) {
> >                       kbuf->image->start -= sechdrs[i].sh_addr;
> >                       kbuf->image->start += kbuf->mem + offset;
> >               }
> >
> > --
> > 2.40.0.348.gf938b09366-goog-b4-0.11.0-dev-696ae
> >
> > _______________________________________________
> > kexec mailing list
> > kexec@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/kexec
> >


--
Ricardo Ribalda