Re: [PATCH v2] elf: Remove ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA

[Fangrui Song <maskray@google.com>]

On 2022-06-01, Szabolcs Nagy wrote:
>The 05/31/2022 21:50, Fangrui Song wrote:
>> If an executable has copy relocations for extern protected data, that
>> can only work if the library containing the definition is built with
>> assumptions (a) the compiler emits GOT-generating relocations (b) the
>> linker produces R_*_GLOB_DAT instead of R_*_RELATIVE.  Otherwise the
>> library uses its own definition directly and the executable accesses a
>> stale copy.  Note: the GOT relocations defeat the purpose of protected
>> visibility as an optimization, but allow rtld to make the executable and
>> library use the same copy when copy relocations are present, but it
>> turns out this never worked perfectly.
>>
>> ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA has strange semantics when both
>> a.so and b.so define protected var and the executable copy relocates
>> var: b.so accesses its own copy even with GLOB_DAT.  The behavior change
>> is from commit 62da1e3b00b51383ffa7efc89d8addda0502e107 (x86) and then
>> copied to nios2 (ae5eae7cfc9c4a8297ff82ec6b794faca1976ecc) and arc
>> (0e7d930c4c11de896fe807f67fa1eb756c9c1e05).
>>
>> Without ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA, b.so accesses the copy
>> relocated data like a.so.
>>
>> It's extremely unlikely anyone relies on the
>> ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA behavior, so let's remove it.
>>
>> --
>> Changes from v1:
>> * Reword commit message as suggested by Szabolcs Nagy
>
>Please document the interposition change or fix it.

Does this paragraph count as an explanation?
"Without ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA, b.so accesses the copy relocated data like a.so."

Or, what other documentation on the interposition behavior do you expect?

>That is important as it affects binaries without copy relocation.