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Date:   Tue, 17 Jan 2023 11:22:45 -0800
From:   Lucas De Marchi <lucas.demarchi@intel.com>
To:     Michal =?utf-8?B?U3VjaMOhbmVr?= <msuchanek@suse.de>
CC:     Gary Guo <gary@garyguo.net>, Kees Cook <keescook@chromium.org>,
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Subject: Re: [PATCH] modpost: support arbitrary symbol length in modversion
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On Tue, Jan 17, 2023 at 06:51:44PM +0100, Michal Such?nek 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.

Lucas De Marchi

>
>Thanks
>
>Michal