On Mon, 12 Mar 2018 15:42:18 +0800
Dave Young <dyo...@redhat.com> wrote:

> Hi Philipp,
> On 03/09/18 at 11:02am, Philipp Rudo wrote:
> > Hi Dave,
> > 
> > 
> > On Fri, 9 Mar 2018 11:14:20 +0800
> > Dave Young <dyo...@redhat.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 <pr...@linux.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.  
> 
> Looks better to add above comment here.

Ok, it's added. I'll wait what Ingo says about the string.c issue befor
sending around v2. 

Thanks
Philipp

> > 
> > 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
> > >   
> >   
> 
> Thanks
> Dave
> 

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