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In the accessor functions ld*_he_p() and st*_he_p() we use memcpy() to perform a load or store to a pointer which might not be aligned for the size of the type. We rely on the compiler to optimize this memcpy() into an efficient load or store instruction where possible. This is required for good performance, but at the moment it is also required for correct operation, because some users of these functions require that the access is atomic if the pointer is aligned, which will only be the case if the compiler has optimized out the memcpy(). (The particular example where we discovered this is the virtio vring_avail_idx() which calls virtio_lduw_phys_cached() which eventually ends up calling lduw_he_p().) Unfortunately some compile environments, such as the fortify-source setup used in Alpine Linux, define memcpy() to a wrapper function in a way that inhibits this compiler optimization. The correct long-term fix here is to add a set of functions for doing atomic accesses into AddressSpaces (and to other relevant families of accessor functions like the virtio_*_phys_cached() ones), and make sure that callsites which want atomic behaviour use the correct functions. In the meantime, switch to using __builtin_memcpy() in the bswap.h accessor functions. This will make us robust against things like this fortify library in the short term. In the longer term it will mean that we don't end up with these functions being really badly-performing even if the semantics of the out-of-line memcpy() are correct. Reported-by: Fernando Casas Schössow <casasfernando@outlook.com> Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Message-Id: <20190318112938.8298-1-peter.maydell@linaro.org> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> |
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