[llvm-dev] [lld][ELF] Add option to make .dynamic read only

[Jake Ehrlich via llvm-dev]

Your understanding is correct. It saves a small number of physical pages.
It's strictly better even if it's only a little bit better.

On Tue, May 16, 2017, 5:14 PM Rui Ueyama <ruiu at google.com> wrote:

> Hi Jake,
>
> Thank you for sending this to llvm-dev!
>
> If I understand correctly, your motivation to add an option to make
> .dynamic sections read-only is to share .dynamic sections in memory so that
> each process executing the same file doesn't have to have a separate copy
> of a .dynamic section. But you would usually save just one page or two by
> doing that because .dynamic sections are usually very small. Even if they
> are large, they could be shared between processes by copy-on-write. What am
> I missing?
>
> On Tue, May 16, 2017 at 4:31 PM, Jake Ehrlich via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>> This is a proposal to add an option to lld that forces .dynamic sections
>> to be read-only.
>>
>> The .dynamic section is almost read-only except for the DT_DEBUG entry
>> which requires the dynamic linker to modify a word. MIPS has long since had
>> a solution to this using the DT_MIPS_RLD_MAP entry to give a pointer to
>> another section which is writable. It would be nice to have this
>> functionality on other targets as well however. Right now many dynamic
>> linkers do not support this layer of indirection so this can't happen by
>> default, it must be an option. Currently .dynamic gets mapped to a
>> PT_GNU_RELRO section. PT_GNU_RELRO sections are not shared between
>> instances of a executable however. It would be a nice optimization to share
>> the .dynamic section of an application where possible. In particular we
>> would like to have this option for Fuchsia. Other dynamic linkers, like
>> musl, already have some support for this on non-MIPS targets as seen here:
>> http://git.musl-libc.org/cgit/musl/tree/ldso/dynlink.c#n1629. In
>> particular the DT_DEBUG_INDIRECT entry is recognized.
>>
>> This change would refactor code that already exists for MIPS specific
>> .dynamic sections and make them available to other targets. Mostly this
>> would just require renaming some things and moving a few lines around. The
>> biggest part of this change would be to add the flag. I propose it be a -z
>> keyword called "rodynamic".
>>
>> I have already made a patch for this change which can be seen here:
>> https://reviews.llvm.org/D33251
>>
>> Best,
>> Jake
>>
>> _______________________________________________
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>> llvm-dev at lists.llvm.org
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>>
>>
>
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