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

[Gary Guo <gary@garyguo.net>]

On Tue, 17 Jan 2023 11:22:45 -0800
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?

Yeah, that's what I imagined how it could be implemented when I said
"short symbols can still be found by old versions of kmod, but not long
symbols".

> 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.

That format isn't really elegant IMO. And symbol length can vary a lot,
having all symbols dictated by the longest symbol doesn't sound a good
approach.

> And the additional 0's should compress well
> so I'm not sure the additional size is that much relevant here.

I am not sure why compression is mentioned here. I don't think section
in .ko files are compressed.

(sorry forget to reply-all, re-send email to the list)

Best,
Gary