Messages in this thread Patch in this message | | | Date | Fri, 5 Apr 2019 01:58:53 GMT | From | George Spelvin <> | Subject | [PATCH v3] ubsan: Avoid unnecessary 128-bit shifts |
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If CONFIG_ARCH_SUPPORTS_INT128, s_max is 128 bits, and variable sign-extending shifts of such a double-word data type are a non-trivial amount of code and complexity. Do a single-word sign-extension *before* the cast to (s_max), greatly simplifying the object code.
Rasmus Villemoes suggested using sign_extend* from <linux/bitops.h>.
On s390 (and perhaps some other arches), gcc implements variable 128-bit shifts using an __ashrti3 helper function which the kernel doesn't provide, causing a link error. In that case, this patch is a prerequisite for enabling INT128 support. Andrey Ryabinin has gven permission for any arch that needs it to cherry-pick it so they don't have to wait for ubsan to be merged into Linus' tree.
We *could*, alternatively, implement __ashrti3, but that becomes dead as soon as this patch is merged, so it seems like a waste of time and its absence discourages people from adding inefficient code. Note that the shifts in <math64.h> (unsigned, and by a compile-time constant amount) are simpler and generated inline.
Signed-off-by: George Spelvin <lkml@sdf.org> Acked-By: Andrey Ryabinin <aryabinin@virtuozzo.com> Feedback-from: Rasmus Villemoes <linux@rasmusvillemoes.dk> Cc: linux-s390@vger.kernel.org Cc: Heiko Carstens <heiko.carstens@de.ibm.com> --- include/linux/bitops.h | 7 +++++++ lib/ubsan.c | 13 +++++-------- 2 files changed, 12 insertions(+), 8 deletions(-)
v3: Added sign_extend_long() to sign_extend{32,64} in <linux/bitops.h>. Used sign_extend_long rather than hand-rolling sign extension. Changed to more uniform if ... else if ... else ... structure. v2: Eliminated redundant cast to (s_max). Rewrote commit message without "is this the right thing to do?" verbiage. Incorporated ack from Andrey Ryabinin.
diff --git a/include/linux/bitops.h b/include/linux/bitops.h index 705f7c442691..8d33c2bfe6c5 100644 --- a/include/linux/bitops.h +++ b/include/linux/bitops.h @@ -157,6 +157,13 @@ static inline __s64 sign_extend64(__u64 value, int index) return (__s64)(value << shift) >> shift; } +static inline long sign_extend_long(unsigned long value, int index) +{ + if (sizeof(value) == 4) + return sign_extend32(value); + return sign_extend64(value); +} + static inline unsigned fls_long(unsigned long l) { if (sizeof(l) == 4) diff --git a/lib/ubsan.c b/lib/ubsan.c index e4162f59a81c..24d4920317e4 100644 --- a/lib/ubsan.c +++ b/lib/ubsan.c @@ -88,15 +88,12 @@ static bool is_inline_int(struct type_descriptor *type) static s_max get_signed_val(struct type_descriptor *type, unsigned long val) { - if (is_inline_int(type)) { - unsigned extra_bits = sizeof(s_max)*8 - type_bit_width(type); - return ((s_max)val) << extra_bits >> extra_bits; - } + if (is_inline_int(type)) + return sign_extend_long(val, type_bit_width(type) - 1); - - if (type_bit_width(type) == 64) + else if (type_bit_width(type) == 64) return *(s64 *)val; - - return *(s_max *)val; + else + return *(s_max *)val; } static bool val_is_negative(struct type_descriptor *type, unsigned long val) -- 2.20.1
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