Messages in this thread | | | Date | Fri, 02 Mar 2007 13:06:46 -0800 | From | John Reiser <> | Subject | Re: + fully-honor-vdso_enabled.patch added to -mm tree |
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Oleg Nesterov wrote: > John Reiser wrote:
>>+ switch (vdso_enabled) { >>+ case 0: /* none */ >>+ return 0; > > > This means we don't initialize mm->context.vdso and ->sysenter_return. > > Is it ok? For example, setup_rt_frame() uses VDSO_SYM(&__kernel_rt_sigreturn), > sysenter_past_esp pushes ->sysenter_return on stack.
Paul Mundt has commented on setup_rt_frame() and provided a patch which bullet-proofs that area. I will include that patch into the next revision.
The value of ->sysenter_return is interpreted in user space by the sysexit instruction; nobody else cares what the value is. The kernel is not required to provide a good value when vdso_enabled is zero, because the kernel has not told the process that sysenter is valid (by setting AT_SYSINFO.) The kernel requires specific register values for sysenter+sysexit and these values may change at the whim of the kernel, so correct code must follow the kernel's protocol. glibc uses sysenter only when AT_SYSINFO is present. User code can screw up even when vdso_enabled is non-zero, by overwriting or re- mapping the vdso page (clobber memory at the destination of sysexit.)
Both context.vdso and sysenter_return could be set to zero whenever vdso_enabled is zero; those two values might even be defaulted. I'll add such a change to the next revision of the patch, if you'll defend it against claims of "unnecessary code."
> > Note also that load_elf_binary does > > arch_setup_additional_pages() > create_elf_tables() > > , looks like application can crash after exec if vdso_enabled changes from 0 > to 1 in between.
Correct. Changing vdso_enabled from 0 to non-zero must be prepared to lose this race if it is not prevented. Ordinarily it won't matter because the administrator will perform such changes at a "quiet" time.
-- John Reiser, jreiser@BitWagon.com - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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