Hello, This is my first patch to the Linux kernel. I hope sending it via gmail is OK? I have attached the git-generated patch file. This is a simple fix to a problem I encountered when I tried to build CPython. Some digging suggested that the problem is actually with kernel header files. Basically the problem is that when calculating `HUGETLB_FLAG_ENCODE_16GB` we essentially run (34 << 26). That's undefined behaviour in C, because C tries to do these calculations as signed ints, but signed signed ints don't have enough bits. The fix is to cast to unsigned first: ((unsigned) 32 << 26). Unsigned ints have enough bits, at least on 32 bit systems and above. I suspect the way the kernel build uses GCC and Clang this is probably not a problem, but the headers are used by outside projects (like CPython), and for that it's probably best to stick to the C standard, when it's easy to do so. Thanks, Matthias. P.S. Please pardon, if you received this email a second time, I had some trouble getting GMail to send in plain text.
From 67ba739770997641648ef07f9d5ddb46802bdccf Mon Sep 17 00:00:00 2001 From: Matthias Goergens <matthias.goergens@gmail.com> Date: Sat, 3 Sep 2022 14:28:21 +0800 Subject: [PATCH] tools/headers: Fix undefined behaviour (34 << 26) Left-shifting past the size of your datatype is undefined behaviour in C. The literal 34 gets the type `int`, and that one is not big enough to be left shifted by 26 bits. An `unsigned` is long enough (on any machine that has at least 32 bits for their ints.) For uniformity, we add the (unsigned) cast everywhere. But it's only really needed for HUGETLB_FLAG_ENCODE_16GB. Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com> --- include/uapi/asm-generic/hugetlb_encode.h | 26 +++++++++++----------- tools/include/asm-generic/hugetlb_encode.h | 26 +++++++++++----------- 2 files changed, 26 insertions(+), 26 deletions(-) diff --git a/include/uapi/asm-generic/hugetlb_encode.h b/include/uapi/asm-generic/hugetlb_encode.h index 4f3d5aaa11f5..afa9f44cc2e7 100644 --- a/include/uapi/asm-generic/hugetlb_encode.h +++ b/include/uapi/asm-generic/hugetlb_encode.h @@ -20,18 +20,18 @@ #define HUGETLB_FLAG_ENCODE_SHIFT 26 #define HUGETLB_FLAG_ENCODE_MASK 0x3f -#define HUGETLB_FLAG_ENCODE_16KB (14 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_64KB (16 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_512KB (19 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_1MB (20 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_2MB (21 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_8MB (23 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_16MB (24 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_32MB (25 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_256MB (28 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_512MB (29 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_1GB (30 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_2GB (31 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_16GB (34 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_16KB ((unsigned) 14 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_64KB ((unsigned) 16 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_512KB ((unsigned) 19 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_1MB ((unsigned) 20 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_2MB ((unsigned) 21 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_8MB ((unsigned) 23 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_16MB ((unsigned) 24 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_32MB ((unsigned) 25 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_256MB ((unsigned) 28 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_512MB ((unsigned) 29 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_1GB ((unsigned) 30 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_2GB ((unsigned) 31 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_16GB ((unsigned) 34 << HUGETLB_FLAG_ENCODE_SHIFT) #endif /* _ASM_GENERIC_HUGETLB_ENCODE_H_ */ diff --git a/tools/include/asm-generic/hugetlb_encode.h b/tools/include/asm-generic/hugetlb_encode.h index 4f3d5aaa11f5..afa9f44cc2e7 100644 --- a/tools/include/asm-generic/hugetlb_encode.h +++ b/tools/include/asm-generic/hugetlb_encode.h @@ -20,18 +20,18 @@ #define HUGETLB_FLAG_ENCODE_SHIFT 26 #define HUGETLB_FLAG_ENCODE_MASK 0x3f -#define HUGETLB_FLAG_ENCODE_16KB (14 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_64KB (16 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_512KB (19 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_1MB (20 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_2MB (21 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_8MB (23 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_16MB (24 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_32MB (25 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_256MB (28 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_512MB (29 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_1GB (30 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_2GB (31 << HUGETLB_FLAG_ENCODE_SHIFT) -#define HUGETLB_FLAG_ENCODE_16GB (34 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_16KB ((unsigned) 14 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_64KB ((unsigned) 16 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_512KB ((unsigned) 19 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_1MB ((unsigned) 20 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_2MB ((unsigned) 21 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_8MB ((unsigned) 23 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_16MB ((unsigned) 24 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_32MB ((unsigned) 25 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_256MB ((unsigned) 28 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_512MB ((unsigned) 29 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_1GB ((unsigned) 30 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_2GB ((unsigned) 31 << HUGETLB_FLAG_ENCODE_SHIFT) +#define HUGETLB_FLAG_ENCODE_16GB ((unsigned) 34 << HUGETLB_FLAG_ENCODE_SHIFT) #endif /* _ASM_GENERIC_HUGETLB_ENCODE_H_ */ -- 2.37.3
