[PATCH] D66426: [lld] Enable a watermark of loadable sections to be generated and placed in a note section

Wed Nov 20 06:23:08 PST 2019

chrisjackson added a comment.

In D66426#1664187 <https://reviews.llvm.org/D66426#1664187>, @MaskRay wrote:

> This patch introduces a new ELF extension with a new tag: NT_LLVM_WATERMARK. Last times such ELF extensions were made accompanying RFCs were posted:
>
> https://lists.llvm.org/pipermail/llvm-dev/2019-February/130583.html
>  https://lists.llvm.org/pipermail/llvm-dev/2019-March/131004.html
>
> I still have some concerns about the extension, especially its generality on the widely used ELF platforms.
>
> > We would like to have two PT_NOTEs, one for use by the OS and another for use by tooling. This is achieved by linker scripts. The second PT_NOTE is outside of any PT_LOAD and this is where the watermark would be housed. As described above, a required property of the watermark is that it can be recalculated by an external tool to infer whether or not the loadable parts of the ELF have been modified post-link. By having the watermark outside of any PT_LOAD, it is simpler for the external tool to recalculate. For a similar reason, it would actually be better in our case to have the watermark calculated after the build ID value has been "filled-in", as the build ID is inside a PT_LOAD.
>
> Sorry, I am not following. Do you have a concrete readelf -Sl dump to help my understanding?
>  By linking a trivial executable with `ld.lld a.o --watermark -o a`, what I see is:
>
>    PHDR           0x000040 0x0000000000200040 0x0000000000200040 0x000118 0x000118 R   0x8
>    LOAD           0x000000 0x0000000000200000 0x0000000000200000 0x000174 0x000174 R   0x1000
>    GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0
>    NOTE           0x000158 0x0000000000200158 0x0000000000200158 0x00001c 0x00001c R   0x4
>   
>   Section to Segment mapping:
>    Segment Sections...
>     00
>     01     .note.llvm.watermark
>     02
>     03     .note.llvm.watermark
>   
>
> .note.llvm.watermark is still included in a PT_LOAD segment. I think you probably meant:
>
> - .note.gnu.build-id is included in a PT_LOAD and a PT_NOTE
> - .note.llvm.watermark is included in another PT_NOTE
>
>   There is also a question why .note.llvm.watermark should be flagged SHF_ALLOC if it is not supposed to be inspected at runtime.

I'll provide an readelf -Sl dump that will illustrate the note to segment mapping.

In D66426#1752900 <https://reviews.llvm.org/D66426#1752900>, @ruiu wrote:

> I think this feature is worth more attention. There may be someone who wants to start using this once it's landed, and I'd make sure that we satisfy their needs. Do you mind if I ask you to start a thread on llvm-dev to propose this feature? I think that even a comment like "we'll use this" is valuable. tlike "

Of course, I will post to llvm-dev.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D66426/new/

https://reviews.llvm.org/D66426