Subject: bug#30820: Chunked store references in compiled code break grafting (again)

From: [email-masked] (Ludovic Courtès)
Date: Mon, 19 Mar 2018 23:34:38 +0100

Hi Mark,

Mark H Weaver <[email-masked]> skribis:

> [email-masked] (Ludovic Courtès) writes:
>
>> The recently added glibc grafts triggered issues that, in the end, show
>> the return of <http://bugs.gnu.org/24703> (“Store references in 8-byte
>> chunks in compiled code”).
>
> I think that we should generalize our reference scanning and grafting
> code to support store references broken into pieces, as long as each
> piece containing part of the hash is at least 8 bytes long.
>
> Here's my preliminary proposal:
>
> (1) The reference scanner should recognize any 8-byte substring of a
>     hash as a valid reference to that hash.
>
> (2) To enable reliable grafting of chunked references, we should impose
>     the following new restrictions: (a) the store prefix must be at
>     least 6 bytes, (b) grafting can change only the hash, not the
>     readable part of the store name, and (c) the readable part of the
>     store name must be at least 6 bytes.
>
> (3) The grafter should recognize and replace any 8-byte subsequence of
>     the absolute store file name.

I’m quite reluctant because it would add complexity, it will probably
slow things down, and yet it may not handle all the cases, as Danny
suggests.

Mind you, the GCC patches are not perfect either, but they’re relatively
easy to deal with (well, so far at least).  In theory we would need
similar patches for LLVM and maybe a couple other native compilers,
though, which is obviously a downside, although we haven’t had any
problems so far.

WDYT?

Ludo’.