On Fri, Feb 21, 2025 at 4:11 PM Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx> wrote: > > On 2025-02-20 at 00:31:08 +0100, Andrey Konovalov wrote: > >On Tue, Feb 18, 2025 at 9:20 AM Maciej Wieczor-Retman > ><maciej.wieczor-retman@xxxxxxxxx> wrote: > >> > >> On x86, generic KASAN is setup in a way that needs a single > >> KASAN_SHADOW_OFFSET value for both 4 and 5 level paging. It's required > >> to facilitate boot time switching and it's a compiler ABI so it can't be > >> changed during runtime. > >> > >> Software tag-based mode doesn't tie shadow start and end to any linear > >> addresses as part of the compiler ABI so it can be changed during > >> runtime. > > > >KASAN_SHADOW_OFFSET is passed to the compiler via > >hwasan-mapping-offset, see scripts/Makefile.kasan (for the INLINE > >mode). So while we can change its value, it has to be known at compile > >time. So I don't think using a runtime constant would work. > > I don't know about arm64, but this doesn't seem to work right now on x86. You mean it _does_ seem to work? Or otherwise if runtime constant doesn't work on x86, then we shouldn't use it? > I > think I recall that hwasan-mapping-offset isn't implemented on the x86 LLVM or > something like that? I'm sure I saw some note about it a while ago on the > internet but I couldn't find it today. In LLVM sources, ShadowBase gets calculated [1] based on Mapping.Offset [2], which is in turn taken [3] from hwasan-mapping-offset [4]. And then ShadowBase is used to calculate [5] the shadow memory address. All of this happens in the common code, so this should affect both x86 and arm64. [1] https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L1305 [2] https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L761 [3] https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L1863 [4] https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L171 [5] https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L899 > > Anyway if KASAN_SHADOW_OFFSET is not set at compile time it defaults to nothing > and just doesn't get passed into kasan-params a few lines below. I assume that > result seems a little too makeshift for runtime const to make sense here? Sorry, I don't understand this question. If hwasan-mapping-offset is not set properly, then in the inline instrumentation mode, the compiler won't generate the right instructions to calculate the shadow memory address.
