Messages in this thread | | | Date | Fri, 13 Jun 2014 09:03:02 -0700 | From | "H. Peter Anvin" <> | Subject | Re: vdso feature requests from the Go people |
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On 06/13/2014 08:34 AM, Andy Lutomirski wrote: > > I'm only aware of two implementations that work like that: glibc and > musl. AFAIK neither one even tries to use the vdso when statically > linked. IIRC, Bionic doesn't support the vdso at all, and Go has the > present issue. >
I would expect uClibc to behave similarly. Bionic does, indeed, not support the vdso, but that is not for the lack of a linker but is really a shortcoming in Bionic.
> And ELF parsing is a giant mess. Currently the vdso doesn't use > DT_GNU_HASH (easy to fix) but no one can safely rely on DT_GNU_HASH > being there, and DT_GNU_HASH isn't actually easier to parse.
Right... and the vdso is small enough that the performance doesn't matter. However, we probably *ought* to publish DT_GNU_HASH data.
>>> 2. Go uses a segmented stack, and the vdso is quite unfriendly for >>> segmented stack. If we can get compiler support, is there a >>> reasonable way that we could advertise the maximum stack usage of each >>> vdso entry point? >> >> I suspect an easier way to do that would just be to define a maximum >> stack usage for *any* vdso entry point, and then enable the gcc stack >> depth warning (perhaps even with Werror)... we can do this now. > > I can imagine this causing lots of pain when gcc 4.11 comes out with > some issue that blows up the stack usage. Or when akpm compiles on > Fedora Core 6 using some ancient toolchain that spills every local > variable three or four times and assigns every possible inline > function its own non-overlapping stack range. > > My copy of gcc supports -fstack-usage, which seems like an easyish way > to obtain the information. I'm not entirely sure whether > -fstack-usage refers to the whole call tree or just to the specific > function.
There are issues either way. However, most vdso code doesn't use much stack at all, and it seems reasonable to put a (conservative) cap on it as a matter of policy.
-hpa
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