Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 19 Jan 2023 11:02:08 -0800 | From | Lucas De Marchi <> | Subject | Re: [PATCH] modpost: support arbitrary symbol length in modversion |
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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
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