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

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Date:   Thu, 19 Jan 2023 16:18:57 +0100
From:   Michal Suchánek <msuchanek@...e.de>
To:     Gary Guo <gary@...yguo.net>
Cc:     Lucas De Marchi <lucas.demarchi@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        linux-kbuild@...r.kernel.org,
        Wedson Almeida Filho <wedsonaf@...gle.com>,
        Joel Stanley <joel@....id.au>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Nicolas Schier <nicolas@...sle.eu>,
        rust-for-linux@...r.kernel.org,
        Guo Zhengkui <guozhengkui@...o.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, Julia Lawall <Julia.Lawall@...ia.fr>,
        Luis Chamberlain <mcgrof@...nel.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-modules@...r.kernel.org
Subject: Re: [PATCH] modpost: support arbitrary symbol length in modversion

On Thu, Jan 19, 2023 at 03:09:36PM +0000, Gary Guo wrote:
> On Tue, 17 Jan 2023 11:22:45 -0800
> Lucas De Marchi <lucas.demarchi@...el.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@...el.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.

There is the option to compress the whole .ko files, and it's commonly
used.

Thanks

Michal