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

[Lucas De Marchi <lucas.demarchi@intel.com>]

On Wed, Jan 18, 2023 at 04:01:29PM +0900, Masahiro Yamada wrote:
>On Wed, Jan 18, 2023 at 4:23 AM Lucas De Marchi
><lucas.demarchi@intel.com> 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@intel.com> 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.

I like this option. It would be better than the current one.

Lucas De Marchi

>
>
>
>
>
>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