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    SubjectRe: [PATCH] modpost: support arbitrary symbol length in modversion
    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.





    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

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