Messages in this thread | | | Subject | Re: [PATCH] module/ksymtab: use 64-bit relative reference for target symbol | From | Ard Biesheuvel <> | Date | Thu, 23 May 2019 09:41:40 +0100 |
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On 5/22/19 5:28 PM, Ard Biesheuvel wrote: > > > On 5/22/19 4:02 PM, Ard Biesheuvel wrote: >> The following commit >> >> 7290d5809571 ("module: use relative references for __ksymtab entries") >> >> updated the ksymtab handling of some KASLR capable architectures >> so that ksymtab entries are emitted as pairs of 32-bit relative >> references. This reduces the size of the entries, but more >> importantly, it gets rid of statically assigned absolute >> addresses, which require fixing up at boot time if the kernel >> is self relocating (which takes a 24 byte RELA entry for each >> member of the ksymtab struct). >> >> Since ksymtab entries are always part of the same module as the >> symbol they export (or of the core kernel), it was assumed at the >> time that a 32-bit relative reference is always sufficient to >> capture the offset between a ksymtab entry and its target symbol. >> >> Unfortunately, this is not always true: in the case of per-CPU >> variables, a per-CPU variable's base address (which usually differs >> from the actual address of any of its per-CPU copies) could be at >> an arbitrary offset from the ksymtab entry, and so it may be out >> of range for a 32-bit relative reference. >>
(Apologies for the 3-act monologue)
This turns out to be incorrect. The symbol address of per-CPU variables exported by modules is always in the vicinity of __per_cpu_start, and so it is simply a matter of making sure that the core kernel is in range for module ksymtab entries containing 32-bit relative references.
When running the arm64 with kaslr enabled, we currently randomize the module space based on the range of ADRP/ADD instruction pairs, which have a -/+ 4 GB range rather than the -/+ 2 GB range of 32-bit place relative data relocations. So we can fix this by simply reducing the randomization window to 2 GB.
So please disregard this patch (and the followup one against arch/x86/tools)
-- Ard.
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