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![Steve Capper](/assets/img/avatar_default.png)
KASAN_SHADOW_OFFSET is a constant that is supplied to gcc as a command line argument and affects the codegen of the inline address sanetiser. Essentially, for an example memory access: *ptr1 = val; The compiler will insert logic similar to the below: shadowValue = *(ptr1 >> KASAN_SHADOW_SCALE_SHIFT + KASAN_SHADOW_OFFSET) if (somethingWrong(shadowValue)) flagAnError(); This code sequence is inserted into many places, thus KASAN_SHADOW_OFFSET is essentially baked into many places in the kernel text. If we want to run a single kernel binary with multiple address spaces, then we need to do this with KASAN_SHADOW_OFFSET fixed. Thankfully, due to the way the KASAN_SHADOW_OFFSET is used to provide shadow addresses we know that the end of the shadow region is constant w.r.t. VA space size: KASAN_SHADOW_END = ~0 >> KASAN_SHADOW_SCALE_SHIFT + KASAN_SHADOW_OFFSET This means that if we increase the size of the VA space, the start of the KASAN region expands into lower addresses whilst the end of the KASAN region is fixed. Currently the arm64 code computes KASAN_SHADOW_OFFSET at build time via build scripts with the VA size used as a parameter. (There are build time checks in the C code too to ensure that expected values are being derived). It is sufficient, and indeed is a simplification, to remove the build scripts (and build time checks) entirely and instead provide KASAN_SHADOW_OFFSET values. This patch removes the logic to compute the KASAN_SHADOW_OFFSET in the arm64 Makefile, and instead we adopt the approach used by x86 to supply offset values in kConfig. To help debug/develop future VA space changes, the Makefile logic has been preserved in a script file in the arm64 Documentation folder. Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Signed-off-by: Steve Capper <steve.capper@arm.com> Signed-off-by: Will Deacon <will@kernel.org>
27 lines
707 B
Bash
27 lines
707 B
Bash
#!/bin/sh
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# Print out the KASAN_SHADOW_OFFSETS required to place the KASAN SHADOW
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# start address at the mid-point of the kernel VA space
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print_kasan_offset () {
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printf "%02d\t" $1
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printf "0x%08x00000000\n" $(( (0xffffffff & (-1 << ($1 - 1 - 32))) \
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+ (1 << ($1 - 32 - $2)) \
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- (1 << (64 - 32 - $2)) ))
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}
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echo KASAN_SHADOW_SCALE_SHIFT = 3
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printf "VABITS\tKASAN_SHADOW_OFFSET\n"
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print_kasan_offset 48 3
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print_kasan_offset 47 3
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print_kasan_offset 42 3
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print_kasan_offset 39 3
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print_kasan_offset 36 3
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echo
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echo KASAN_SHADOW_SCALE_SHIFT = 4
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printf "VABITS\tKASAN_SHADOW_OFFSET\n"
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print_kasan_offset 48 4
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print_kasan_offset 47 4
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print_kasan_offset 42 4
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print_kasan_offset 39 4
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print_kasan_offset 36 4
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