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Message-Id: <20180309110241.00b0d3f3@ThinkPad>
Date: Fri, 9 Mar 2018 11:02:41 +0100
From: Philipp Rudo <prudo@...ux.vnet.ibm.com>
To: Dave Young <dyoung@...hat.com>
Cc: kexec@...ts.infradead.org, linux-s390@...r.kernel.org,
linux-kernel@...r.kernel.org,
Eric Biederman <ebiederm@...ssion.com>,
Vivek Goyal <vgoyal@...hat.com>,
Michael Ellerman <mpe@...erman.id.au>,
Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
Martin Schwidefsky <schwidefsky@...ibm.com>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
AKASHI Takahiro <takahiro.akashi@...aro.org>
Subject: Re: [PATCH 09/11] kexec_file: Remove mis-use of sh_offset field
Hi Dave,
On Fri, 9 Mar 2018 11:14:20 +0800
Dave Young <dyoung@...hat.com> wrote:
> On 02/26/18 at 04:16pm, Philipp Rudo wrote:
> > The current code uses the sh_offset field in purgatory_info->sechdrs to
> > store a pointer to the current load address of the section. Depending
> > whether the section will be loaded or not this is either a pointer into
> > purgatory_info->purgatory_buf or kexec_purgatory. This is not only a
> > violation of the ELF standard but also makes the code very hard to
> > understand as you cannot tell if the memory you are using is read-only or
> > not.
> >
> > Remove this mis-use and store the offset of the section in
> > pugaroty_info->purgatory_buf in sh_offset.
> >
> > Signed-off-by: Philipp Rudo <prudo@...ux.vnet.ibm.com>
> > ---
> > arch/x86/kernel/machine_kexec_64.c | 10 ++++++----
> > kernel/kexec_file.c | 33 +++------------------------------
> > 2 files changed, 9 insertions(+), 34 deletions(-)
> >
> > diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> > index 51667c8b5c9b..41db74bdc88b 100644
> > --- a/arch/x86/kernel/machine_kexec_64.c
> > +++ b/arch/x86/kernel/machine_kexec_64.c
> > @@ -457,13 +457,15 @@ int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
> > * rel[i].r_offset contains byte offset from beginning
> > * of section to the storage unit affected.
> > *
> > - * This is location to update (->sh_offset). This is temporary
> > - * buffer where section is currently loaded. This will finally
> > - * be loaded to a different address later, pointed to by
> > + * This is location to update. This is temporary buffer
> > + * where section is currently loaded. This will finally be
> > + * loaded to a different address later, pointed to by
> > * ->sh_addr. kexec takes care of moving it
> > * (kexec_load_segment()).
> > */
> > - location = (void *)(section->sh_offset + rel[i].r_offset);
> > + location = pi->purgatory_buf;
> > + location += section->sh_offset;
> > + location += rel[i].r_offset;
> >
> > /* Final address of the location */
> > address = section->sh_addr + rel[i].r_offset;
> > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > index 746b91e46e34..25b44d1a664a 100644
> > --- a/kernel/kexec_file.c
> > +++ b/kernel/kexec_file.c
> > @@ -736,28 +736,6 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
> > pi->ehdr->e_shnum * sizeof(Elf_Shdr));
> > pi->sechdrs = sechdrs;
> >
> > - /*
> > - * We seem to have multiple copies of sections. First copy is which
> > - * is embedded in kernel in read only section. Some of these sections
> > - * will be copied to a temporary buffer and relocated. And these
> > - * sections will finally be copied to their final destination at
> > - * segment load time.
>
> It would be good to keep above part comment somewhere..
there is a comment in include/linux/kexec.h for the purgatory_info->sechdrs
field saying
Temporary, modifiable buffer for sechdrs used for relocation.
This memory can be freed post image load
When you think this is not enough i can add a comment here like
The section headers in kexec_purgatory are read-only. In order to have them
modifiable make a temporary copy.
What do you think.
Thanks
Philipp
> > - *
> > - * Use ->sh_offset to reflect section address in memory. It will
> > - * point to original read only copy if section is not allocatable.
> > - * Otherwise it will point to temporary copy which will be relocated.
> > - *
> > - * Use ->sh_addr to contain final address of the section where it
> > - * will go during execution time.
> > - */
> > - for (i = 0; i < pi->ehdr->e_shnum; i++) {
> > - if (sechdrs[i].sh_type == SHT_NOBITS)
> > - continue;
> > -
> > - sechdrs[i].sh_offset = (unsigned long)pi->ehdr +
> > - sechdrs[i].sh_offset;
> > - }
> > -
> > offset = 0;
> > bss_addr = kbuf->mem + kbuf->bufsz;
> > kbuf->image->start = pi->ehdr->e_entry;
> > @@ -786,17 +764,12 @@ static int kexec_purgatory_setup_sechdrs(struct purgatory_info *pi,
> > kbuf->image->start += kbuf->mem + offset;
> > }
> >
> > - src = (void *)sechdrs[i].sh_offset;
> > + src = (void *)pi->ehdr + sechdrs[i].sh_offset;
> > dst = pi->purgatory_buf + offset;
> > memcpy(dst, src, sechdrs[i].sh_size);
> >
> > sechdrs[i].sh_addr = kbuf->mem + offset;
> > -
> > - /*
> > - * This section got copied to temporary buffer. Update
> > - * ->sh_offset accordingly.
> > - */
> > - sechdrs[i].sh_offset = (unsigned long)dst;
> > + sechdrs[i].sh_offset = offset;
> > offset += sechdrs[i].sh_size;
> > }
> >
> > @@ -1006,7 +979,7 @@ int kexec_purgatory_get_set_symbol(struct kimage *image, const char *name,
> > return -EINVAL;
> > }
> >
> > - sym_buf = (char *)sec->sh_offset + sym->st_value;
> > + sym_buf = (char *)pi->purgatory_buf + sec->sh_offset + sym->st_value;
> >
> > if (get_value)
> > memcpy((void *)buf, sym_buf, size);
> > --
> > 2.13.5
> >
>
> Thanks
> Dave
>
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