Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 10 Mar 2014 21:10:13 -0700 | Subject | Re: [PATCH v2] x86: Remove compat vdso support |
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On Mon, Mar 10, 2014 at 8:09 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > On Mon, Mar 10, 2014 at 7:37 PM, Andy Lutomirski <luto@amacapital.net> wrote: >> >> It does. My patch breaks OpenSuSE 9 when >> CONFIG_ENABLE_VDSO32_BY_DEFAULT=y unless it's overridden by sysctl or >> boot option. > > Oh, I missed that "when =y" part. > > But why do we then want to have that "=y" as an option at all? > > If the situation is that everybody is fine with that being disabled by > default, let's just make it the default. And I'd even be ok with > removing it as an option *entirely*. > > That would seem to be *much* preferable that having an option that > nobody really wants anyway, and where the default value would break > some users. THAT just seems completely insane.
I suspect that a lot of 32-bit Linux users want syscall and/or sysenter, and Stefani certainly wants the fast timing that the vDSO can provide. Also, presumably __kernel_sigreturn serves some purpose :)
The basic issue is that most old glibc versions (and all versions that were ever tagged in a release) work correctly regardless of whether there is a vDSO, and newer glibc versions (since 2004) will take advantage of a vDSO if one exists, but OpenSuSE 9 shipped with a creatively broken version that blows up if presented with a vDSO that's not prelinked to its actual address.
Currently there are three options: sane vDSO, no vDSO, and OpenSuSE 9-compatible vDSO. The latter is a mess to maintain and breaks ASLR (even for users of modern glibc), and having a vDSO is apparently important enough that people are willing to pay to enhance it. The default is OpenSuSE 9-compatible vDSO, which is IMO an odd choice.
ISTM the right solution is to make OpenSuSE 9 users turn off the vDSO (which is a performance hit for them, but not a correctness issue) and let everyone else have a simpler kernel that has no ASLR issues.
I'm a bit heartened by the fact that the failure mode on OpenSuSE 9 is a rather distinctive and easily Googlable message. Most of the hits offer abi.vsyscall32=0 or vdso=0 as suggestions, both of which continue to work with my patch.
--Andy
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