Messages in this thread | | | Date | Thu, 13 Dec 2012 16:36:22 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] Add VDSO time function support for x86 32-bit kernel |
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On 12/13/2012 04:20 PM, Andy Lutomirski wrote: >> >> Whatever data you need you can just map it into the vdso range. There >> really shouldn't be anything special about that at all. >> >> The fixmap stuff is an x86-64 legacy that you don't have to worry about, >> obviously. > > The fixmap page is new. It's not ABI -- the layout can freely change, > but the vdso needs to be able to see. It would make no sense to cow > that page, and it would be more complicated to make it be part of the > (64-bit) vdso, so I left it as a fixmap when I did the vvar cleanups. >
Well, the vsyscall fixmap is an ABI. But just because a page is mapped into userspace doesn't mean cow. Think of it as a device mmap, or an mmap of a shared file.
> For compat mode, though, I don't think it can be in the fixmap unless > there are segmentation games that I think are impossible, or unless > the vdso wants to do a far call (which I would guess is not much > faster than a system call, although I haven't benchmarked it). This > is because there are no addresses at all that can be seen from 32-bit > code segments and that are in the kernel address range.
Yes, you'd have to nip into 64-bit mode which is not really practical.
> What you could do is probably arrange (using some linker script magic) > for a symbol to exist that points at the page *before* the vdso > starts. Then just map the vvar page there when starting a compat > task. You could then address it using a normal symbol reference by > tweaking the vvar macro. (I think this'll access it via the GOT.) > Alternatively, you could just pick an absolute address -- the page is > NX, so you don't really need to worry about randomization.
IMO it seems this is making it way more complicated than it is. Just make sure you have a section in the vdso where you can map in a data page with the symbols in the right offsets. Extra points for doing magic so that it is at the beginning or end, but I think that might be harder than necessary.
-hpa
-- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
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