Re: Multiboot ELF segment handling patch

[Daniel Kiper]

Hi Alexander,

On Sat, Apr 14, 2018 at 12:07:29AM +0200, Alexander Boettcher wrote:
> Hello,
>
> On 06.04.2018 14:28, Daniel Kiper wrote:
> > On Thu, Mar 29, 2018 at 11:20:37AM +0200, Alexander Boettcher wrote:
> >
> >> Can you please have a look and check regarding what should/could be
> >> changed to get it upstream? We did not test the dynamic relocation part,
> >
> > Sure thing, please take a look below...
> >
> >> since we have no such (kernel) setup. Thanks in advance.
> >
> > You can use Xen (git://xenbits.xen.org/xen.git) for tests.
> > Just compile hypervisor without any tools and use xen.gz.
> > It produces nice output and you can see it is relocated or not.
> > Of course you have to use Multiboot2 protocol.
>
> Thanks, I managed to setup it. Based on your comments and on the Xen
> test case I had to re-work the patch, so that it now works for
> relocation and non-relocation kernels.
>
> Please review again.
> > However, this does not mean that I will not take this patch. Though
> > it requires some tweaking.
> >
> > First of all, lack of SOB.
>
> What is a SOB ?

Signed-off-by: Alexander Boettcher <address@hidden>

Next time please use git format-patch/send-email to send the patches.

> From c37727840be8aeb81ea4bbc95ac09a1b1e3d3658 Mon Sep 17 00:00:00 2001
> From: Alexander Boettcher <address@hidden>
> Date: Fri, 13 Apr 2018 23:37:01 +0200
> Subject: [PATCH] mbi: use per segment a separate relocator chunk
>
> if elf is non relocatable.
>
> If the segments are not dense packed, the original code set up a huge
> relocator chunk comprising all segments.
>
> During the final relocation in grub_relocator_prepare_relocs, the chunk
> is moved to its desired place and overrides memory which are actually not
> covered/touched by the specified segments.
>
> The overriden memory may contain device memory (vga text mode e.g.), which
> leads to strange boot behaviour.

Have you been able to take a look at memory allocator/relocator code why
this happened?

> ---
>  grub-core/loader/multiboot_elfxx.c | 56 
> ++++++++++++++++++++++++--------------
>  1 file changed, 35 insertions(+), 21 deletions(-)
>
> diff --git a/grub-core/loader/multiboot_elfxx.c 
> b/grub-core/loader/multiboot_elfxx.c
> index 67daf59..d936223 100644
> --- a/grub-core/loader/multiboot_elfxx.c
> +++ b/grub-core/loader/multiboot_elfxx.c
> @@ -57,9 +57,9 @@ CONCAT(grub_multiboot_load_elf, XX) (mbi_load_data_t *mld)
>    char *phdr_base;
>    grub_err_t err;
>    grub_relocator_chunk_t ch;
> -  grub_uint32_t load_offset, load_size;
> +  grub_uint32_t load_offset = 0, load_size;
>    int i;
> -  void *source;
> +  void *source = NULL;
>
>    if (ehdr->e_ident[EI_MAG0] != ELFMAG0
>        || ehdr->e_ident[EI_MAG1] != ELFMAG1
> @@ -83,6 +83,7 @@ CONCAT(grub_multiboot_load_elf, XX) (mbi_load_data_t *mld)
>  #define phdr(i)                      ((Elf_Phdr *) (phdr_base + (i) * 
> ehdr->e_phentsize))
>
>    mld->link_base_addr = ~0;
> +  mld->load_base_addr = ~0;
>
>    /* Calculate lowest and highest load address.  */
>    for (i = 0; i < ehdr->e_phnum; i++)
> @@ -108,27 +109,19 @@ CONCAT(grub_multiboot_load_elf, XX) (mbi_load_data_t 
> *mld)
>                                             mld->min_addr, mld->max_addr - 
> load_size,
>                                             load_size, mld->align ? 
> mld->align : 1,
>                                             mld->preference, 
> mld->avoid_efi_boot_services);
> -    }
> -  else
> -    err = grub_relocator_alloc_chunk_addr (GRUB_MULTIBOOT (relocator), &ch,
> -                                        mld->link_base_addr, load_size);
>
> -  if (err)
> -    {
> -      grub_dprintf ("multiboot_loader", "Cannot allocate memory for OS 
> image\n");
> -      return err;
> -    }
> +      if (err)
> +        {
> +          grub_dprintf ("multiboot_loader", "Cannot allocate memory for OS 
> image\n");
> +          return err;
> +        }
>
> -  mld->load_base_addr = get_physical_target_address (ch);
> -  source = get_virtual_current_address (ch);
> +      mld->load_base_addr = get_physical_target_address (ch);
> +      source = get_virtual_current_address (ch);
>
> -  grub_dprintf ("multiboot_loader", "link_base_addr=0x%x, 
> load_base_addr=0x%x, "
> -             "load_size=0x%x, relocatable=%d\n", mld->link_base_addr,
> -             mld->load_base_addr, load_size, mld->relocatable);

I would not drop it completely from ~here. I think that you can display
link_base_addr and relocatable just before current line 102. And you can
put "load_size = highest_load - mld->link_base_addr;" just before current
line 104. Additionally, load_size does not have a real meaning for non
relocatable images right now. Hence, I think that it should not be displayed
for them. Especially if there are more than one PT_LOAD segment.

> -  if (mld->relocatable)
> -    grub_dprintf ("multiboot_loader", "align=0x%lx, preference=0x%x, 
> avoid_efi_boot_services=%d\n",
> -               (long) mld->align, mld->preference, 
> mld->avoid_efi_boot_services);
> +      grub_dprintf ("multiboot_loader", "align=0x%lx, preference=0x%x, 
> avoid_efi_boot_services=%d\n",
> +                  (long) mld->align, mld->preference, 
> mld->avoid_efi_boot_services);
> +    }
>
>    /* Load every loadable segment in memory.  */
>    for (i = 0; i < ehdr->e_phnum; i++)
> @@ -139,7 +132,24 @@ CONCAT(grub_multiboot_load_elf, XX) (mbi_load_data_t 
> *mld)
>         grub_dprintf ("multiboot_loader", "segment %d: paddr=0x%lx, 
> memsz=0x%lx, vaddr=0x%lx\n",
>                       i, (long) phdr(i)->p_paddr, (long) phdr(i)->p_memsz, 
> (long) phdr(i)->p_vaddr);
>
> -       load_offset = phdr(i)->p_paddr - mld->link_base_addr;
> +       if (mld->relocatable)
> +         load_offset = phdr(i)->p_paddr - mld->link_base_addr;
> +       else
> +         {
> +           err = grub_relocator_alloc_chunk_addr (GRUB_MULTIBOOT 
> (relocator), &ch,
> +                                                  phdr(i)->p_paddr, 
> phdr(i)->p_memsz);
> +
> +           if (err)
> +             {
> +               grub_dprintf ("multiboot_loader", "Cannot allocate memory for 
> OS image\n");
> +               return err;
> +             }
> +
> +           if (mld->load_base_addr > get_physical_target_address (ch))
> +             mld->load_base_addr = get_physical_target_address (ch);

              mld->load_base_addr = grub_min (mld->load_base_addr, 
get_physical_target_address (ch));

> +
> +           source = get_virtual_current_address (ch);
> +         }
>
>         if (phdr(i)->p_filesz != 0)
>           {
> @@ -163,6 +173,10 @@ CONCAT(grub_multiboot_load_elf, XX) (mbi_load_data_t 
> *mld)
>          }
>      }
>
> +  grub_dprintf ("multiboot_loader", "link_base_addr=0x%x, 
> load_base_addr=0x%x, "
> +             "load_size=0x%x, relocatable=%d\n", mld->link_base_addr,

Well load_size is not meaningful for non relocatable images. Hmmm...
You can make it more meaningful by summing up all phdr(i)->p_memsz.

Anyway, patch looks much better right now.

Daniel