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

[Masahiro Yamada]

On Tue, Mar 14, 2023 at 11:38 PM Andrea Righi
<andrea.righi@xxxxxxxxxxxxx> wrote:
>
> On Mon, Mar 13, 2023 at 11:09:31PM +0100, Andrea Righi wrote:
> > On Mon, Mar 13, 2023 at 11:02:34PM +0100, Michal Suchánek wrote:
> > > On Mon, Mar 13, 2023 at 10:53:34PM +0100, Andrea Righi wrote:
> > > > On Mon, Mar 13, 2023 at 10:48:53PM +0100, Michal Suchánek wrote:
> > > > > Hello,
> > > > >
> > > > > On Mon, Mar 13, 2023 at 09:32:16PM +0100, Andrea Righi wrote:
> > > > > > On Wed, Jan 11, 2023 at 04:11:51PM +0000, Gary Guo wrote:
> > > > > > > Currently modversion uses a fixed size array of size (64 - sizeof(long))
> > > > > > > to store symbol names, thus placing a hard limit on length of symbols.
> > > > > > > Rust symbols (which encodes crate and module names) can be quite a bit
> > > > > > > longer. The length limit in kallsyms is increased to 512 for this reason.
> > > > > > >
> > > > > > > It's a waste of space to simply expand the fixed array size to 512 in
> > > > > > > modversion info entries. I therefore make it variably sized, with offset
> > > > > > > to the next entry indicated by the initial "next" field.
> > > > > > >
> > > > > > > In addition to supporting longer-than-56/60 byte symbols, this patch also
> > > > > > > reduce the size for short symbols by getting rid of excessive 0 paddings.
> > > > > > > There are still some zero paddings to ensure "next" and "crc" fields are
> > > > > > > properly aligned.
> > > > > > >
> > > > > > > This patch does have a tiny drawback that it makes ".mod.c" files generated
> > > > > > > a bit less easy to read, as code like
> > > > > > >
> > > > > > >     "\x08\x00\x00\x00\x78\x56\x34\x12"
> > > > > > >     "symbol\0\0"
> > > > > > >
> > > > > > > is generated as opposed to
> > > > > > >
> > > > > > >     { 0x12345678, "symbol" },
> > > > > > >
> > > > > > > because the structure is now variable-length. But hopefully nobody reads
> > > > > > > the generated file :)
> > > > > > >
> > > > > > > Link: b8a94bfb3395 ("kallsyms: increase maximum kernel symbol length to 512")
> > > > > > > Link: https://github.com/Rust-for-Linux/linux/pull/379
> > > > > > >
> > > > > > > Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
> > > > > >
> > > > > > Is there any newer version of this patch?
> > > > > >
> > > > > > I'm doing some tests with it, but I'm getting boot failures on ppc64
> > > > > > with this applied (at boot kernel is spitting out lots of oops'es and
> > > > > > unfortunately it's really hard to copy paste or just read them from the
> > > > > > console).
> > > > >
> > > > > Are you using the ELF ABI v1 or v2?
> > > > >
> > > > > v1 may have some additional issues when it comes to these symbol tables.
> > > > >
> > > > > Thanks
> > > > >
> > > > > Michal
> > > >
> > > > I have CONFIG_PPC64_ELF_ABI_V2=y in my .config, so I guess I'm using v2.
> > > >
> > > > BTW, the issue seems to be in dedotify_versions(), as a silly test I
> > > > tried to comment out this function completely to be a no-op and now my
> > > > system boots fine (but I guess I'm probably breaking something else).
> > >
> > > Probably not. You should not have the extra leading dot on ABI v2. So if
> > > dedotify does something that means something generates and then expects
> > > back symbols with a leading dot, and this workaround for ABI v1 breaks
> > > that. Or maybe it is called when it shouldn't.
> >
> > Hm.. I'll add some debugging to this function to see what happens exactly.
>
> Alright I've done more tests across different architectures. My problem
> with ppc64 is that this architecture is evaluating sechdrs[i].sh_size
> using get_stubs_size(), that apparently can add some extra padding, so
> doing (vers + vers->next < end) isn't a reliable check to determine the
> end of the variable array, because sometimes "end" can be greater than
> the last "vers + vers->next" entry.


I am not familiar enough with ppc, so I may be misundering.

Checking the for-loop in module_frob_arch_sections(),
they seem to be orthogonal to me.

dedotify_versions() is only called for the "__versions" section.
get_stubs_size() only affects the ".stubs" section.
I did not get how they are related to each other.


BTW, we decided to not go in this direction in the former discussion.
I am not sure how much effort is needed to track down the issue
in this version.

If we add new sections to keep the backward compatibility
for the current "__versions", this issue may not exist.



> In general I think it'd be more reliable to add a dummy NULL entry at
> the end of the modversion array.
>
> Moreover, I think we also need to enforce struct modversion_info to be
> __packed, just to make sure that no extra padding is added (otherwise it
> may break our logic to determine the offset of the next entry).
>
> > @@ -2062,16 +2066,25 @@ static void add_versions(struct buffer *b, struct module *mod)
> >                               s->name, mod->name);
> >                       continue;
> >               }
> > -             if (strlen(s->name) >= MODULE_NAME_LEN) {
> > -                     error("too long symbol \"%s\" [%s.ko]\n",
> > -                           s->name, mod->name);
> > -                     break;
> > -             }
> > -             buf_printf(b, "\t{ %#8x, \"%s\" },\n",
> > -                        s->crc, s->name);
> > +             name_len = strlen(s->name);
> > +             name_len_padded = (name_len + 1 + 3) & ~3;
> > +
> > +             /* Offset to next entry */
> > +             tmp = TO_NATIVE(8 + name_len_padded);
>
> ^ Here's another issue that I found, you can't use TO_NATIVE() in this
> way, some compilers are complaining (like on s390x this doesn't build).
>
> So we need to do something like:
>
>         /* Offset to next entry */
>         tmp = 8 + name_len_padded
>         tmp = TO_NATIVE(tmp);
>
> I'll do some additional tests with these changes and send an updated
> patch (for those that are interested).
>
> -Andrea



--
Best Regards
Masahiro Yamada