Re: gABI extension proposal: PT_SHMMAP

[Suprateeka R Hegde <hegdesmailbox@gmail.com>]

On 09-Jun-2017 02:33 AM, H. Peter Anvin wrote:
> 
> SHF_NOREAD makes total sense 

Yes but under processor specific extensions. Not generally.

>> 5. Why dont we try to fit this proposal under the bigger one - program
>> properties - proposed by H.J. ? The theme there is to pass such
>> information to runtime.
> 
> I think this is completely orthogonal - that would be a program property
> rather than a per-segment property?

H.J's design, as far as I remember, was extensible to accommodate such
handshakes between linker and loader. I may be wrong though.

> So, a slightly refined proposal (if these numbers conflict with newer
> allocations/proposals please let me know):
> 
> 
> 0x00000008 (2^3) - PF_SHARED
> 
> 	This segment MUST be memory-mapped using MAP_SHARED.  Not
> 	all operating systems need to support this capability.  If this
> 	is not possible due to lack of operating system support or
>  	because the underlying image is not possible to memory-map,
> 	the loading of the image should be aborted with an error.

This is the reason I said it should be OS specific. The above definition
itself is so attached to OS support.

It would make sense to make it part of a generic standard when:

1. There are different loaders provided by different vendors for the same OS
2. (Hypothetically) A loader on one OS is expected to handle/load an ELF
produced by a linker from another OS.

> 0x00001000 (2^12) - SHF_SHARED
> 
> 	This section MUST be made part of a PF_SHARED segment when
> 	linked.

This again is not complete. A non-ambiguous standard should also tell is
it necessary for a gABI compliant ELF linker to mark sections that go
into shared segments, as SHF_SHARED? Similarly, is it necessary for a
gABI compliant linker to mark PT_LOAD that gets shared, as PF_SHARED?

> OPTIONALLY, new reserved section names:
> 
> 	.shrodata	- readonly data section to be mapped shared
> 	.shdata		- writable data section to be mapped shared
> 	.shbss		- zerofill section to be mapped shared

IMHO, these are so trivial that it is only worth to be done under a tool
chain specific option. These are not at all useful in general.

That said,

1. Send the proposal to Generic SYS-V ABI group. Lets see the consensus.
2. I may have to object this if the standards mandates that a gABI
compliant linker *MUST* mark PF_SHARED to mean PT_LOAD as sharable. I am
absolutely fine if marking PF_SHARED is optional -- I will not do it on
HP-UX.

--
Supra