Messages in this thread | | | Date | Wed, 15 Jul 2020 11:26:34 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq |
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----- On Jul 15, 2020, at 10:58 AM, Florian Weimer fweimer@redhat.com wrote:
> * Mathieu Desnoyers: > >> ----- On Jul 15, 2020, at 9:42 AM, Florian Weimer fweimer@redhat.com wrote: >>> * Mathieu Desnoyers: >>> >> [...] >>>> How would this allow early-rseq-adopter libraries to interact with >>>> glibc ? >>> >>> Under all extension proposals I've seen so far, early adopters are >>> essentially incompatible with glibc rseq registration. I don't think >>> you can have it both ways. >> >> The basic question I'm not sure about is whether we are allowed to increase >> the size and alignement of __rseq_abi from e.g. glibc 2.32 to glibc 2.33. > > With the current mechanism (global TLS data symbol), we can do that > using symbol versioning. That means that we can only do this on a > release boundary,
That should not be a problem.
> and that it's incompatible with other libraries which > use an interposing unversioned symbol.
We have the freedom to define the ABI of this shared __rseq_abi symbol right now. Maybe it's not such a good thing to let early adopters use unversioned __rseq_abi symbols.
Let me wrap my head around this scenario then, please let me know if I'm misunderstanding something:
1) glibc 2.32 exposes: __rseq_abi (GLIBC_2.32) with size == 32. __rseq_abi with size == 32 is available as a private symbol within glibc - both symbols alias the same contents.
2) glibc 2.33 exposes: __rseq_abi (GLIBC_2.32) with size == 32. __rseq_abi (GLIBC_2.33) with size == 64. __rseq_abi with size == 64 is available as a private symbol within glibc - the three symbols alias the same contents.
Then what happens if we have a program or preloaded library defining __rseq_abi (without version) with size == 32 loaded with a glibc 2.33 ?
Or what happens if we have a program or preloaded libary defining __rseq_abi (GLIBC_2.32) with size == 32 loaded with a glibc 2.33 ?
I wonder if "GLIBC_*" is the right version namespace for this. Considering that the layout of this structure is defined by the Linux kernel UAPI, maybe we'd want version named as "RSEQ_1.0", "RSEQ_2.0" or something similar.
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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