Messages in this thread | | | Date | Fri, 9 Jun 2023 15:24:38 -0700 | From | Omar Sandoval <> | Subject | Re: [PATCH] x86/unwind/orc: add ELF section with ORC version number |
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On Fri, Jun 09, 2023 at 03:04:30PM -0700, Josh Poimboeuf wrote: > On Thu, Jun 08, 2023 at 03:38:38PM -0700, Omar Sandoval wrote: > > Hi, > > > > As mentioned in the commit message, the motivation for this patch is > > allowing drgn to continue to make use of ORC for kernel stack unwinding. > > > > I want to make it clear that I don't want ORC to be stable ABI. The > > kernel is free to change the format as much as needed, I just need a way > > to detect the change. (drgn already pokes at many kernel internals and > > needs updates for most kernel versions anyways. We have a big test suite > > to catch changes we care about.) > > > > I'm not at all married to (or proud of) this particular implementation; > > I'd be happy to use anything that lets me detect the format version in > > both cases mentioned in the commit message (ELF file or core dump + > > symbol table). > > > > It'd be great if we could get a solution in before 6.4 is released. I > > would've reported this sooner, but I just got back from paternity leave > > last week. > > Hi Omar, > > Peter and I agree this seems fine in principle.
Glad to hear that!
> Though, instead of > using an incrementing version, Peter had the idea to hash the struct, > like: > > awk '/^struct orc_entry {$/ { p=1 } p { print } /^}/ { p=0 }' arch/x86/include/asm/orc_types.h | sha1sum > > That way we don't have to remember to bump the version number, and it > would be more resilient to partial backports in distros. > > Would something like that work for you?
Any sort of unique identifier works for me. One thing that the proposed hash wouldn't catch is if ORC_REG_* or ORC_TYPE_* are ever renumbered (i.e., the meanings of existing values change). It also wouldn't catch if something about the .orc_unwind_ip section changed. But assuming changes like that would be much rarer, it could be handled manually by bumping a "salt" for the hash. E.g., by adding 'BEGIN { print <SALT> }' to the awk script:
awk 'BEGIN { print 1 } /^struct orc_entry {$/ { p=1 } p { print } /^}/ { p=0 }' arch/x86/include/asm/orc_types.h | sha1sum
I'll defer to you guys whether it's easier to remember to bump a version everytime or only in those rare cases.
Thanks, Omar
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