Re: [PATCH] modpost: support arbitrary symbol length in modversion

[Masahiro Yamada]

On Wed, Jan 18, 2023 at 4:23 AM Lucas De Marchi
<lucas.demarchi@xxxxxxxxx> wrote:
>
> On Tue, Jan 17, 2023 at 06:51:44PM +0100, Michal Suchánek wrote:
> >Hello,
> >
> >On Fri, Jan 13, 2023 at 06:18:41PM +0000, Gary Guo wrote:
> >> On Thu, 12 Jan 2023 14:40:59 -0700
> >> Lucas De Marchi <lucas.demarchi@xxxxxxxxx> wrote:
> >>
> >> > On Wed, Jan 11, 2023 at 04:11:51PM +0000, Gary Guo wrote:
> >> > >
> >> > > struct modversion_info {
> >> > >- unsigned long crc;
> >> > >- char name[MODULE_NAME_LEN];
> >> > >+ /* Offset of the next modversion entry in relation to this one. */
> >> > >+ u32 next;
> >> > >+ u32 crc;
> >> > >+ char name[0];
> >> >
> >> > although not really exported as uapi, this will break userspace as this is
> >> > used in the  elf file generated for the modules. I think
> >> > this change must be made in a backward compatible way and kmod updated
> >> > to deal with the variable name length:
> >> >
> >> > kmod $ git grep "\[64"
> >> > libkmod/libkmod-elf.c:  char name[64 - sizeof(uint32_t)];
> >> > libkmod/libkmod-elf.c:  char name[64 - sizeof(uint64_t)];
> >> >
> >> > in kmod we have both 32 and 64 because a 64-bit kmod can read both 32
> >> > and 64 bit module, and vice versa.
> >> >
> >>
> >> Hi Lucas,
> >>
> >> Thanks for the information.
> >>
> >> The change can't be "truly" backward compatible, in a sense that
> >> regardless of the new format we choose, kmod would not be able to decode
> >> symbols longer than "64 - sizeof(long)" bytes. So the list it retrieves
> >> is going to be incomplete, isn't it?
> >>
> >> What kind of backward compatibility should be expected? It could be:
> >> * short symbols can still be found by old versions of kmod, but not
> >>   long symbols;
> >
> >That sounds good. Not everyone is using rust, and with this option
> >people who do will need to upgrade tooling, and people who don't care
> >don't need to do anything.
>
> that could be it indeed. My main worry here is:
>
> "After the support is added in kmod, kmod needs to be able to output the
> correct information regardless if the module is from before/after the
> change in the kernel and also without relying on kernel version."
> Just changing the struct modversion_info doesn't make that possible.
>
> Maybe adding the long symbols in another section? Or ble
> just increase to 512 and add the size to a
> "__versions_hdr" section. If we then output a max size per module,
> this would offset a little bit the additional size gained for the
> modules using rust. And the additional 0's should compress well
> so I'm not sure the additional size is that much relevant here.





I also thought of new section(s) for long symbols.



One idea is to have separate sections for CRCs and symbol names.




section __version_crc:
   0x12345678
   0x23456789
   0x34567890


section __version_sym:
  "very_very_very_very_long_symbol"
  "another_very_very_very_very_very_long_symbol"
  "yet_another_very_very_very_very_very_long_symbol"




You can iterate in each section with this:

   crc += sizeof(u32);
   name += strlen(name) + 1;


Benefits:
  - No next pointer
  - No padding
    - *.mod.c is kept human readable.





BTW, the following is impossible
because the pointer reference to .rodata
is not available at this point?

struct modversion_info {
     u32 crc;
     const char *name:
};



-- 
Best Regards
Masahiro Yamada