* [PATCH babeltrace v2] Fix: bitfield: left shift undefined behavior
@ 2019-05-08 14:39 Mathieu Desnoyers
0 siblings, 0 replies; 5+ messages in thread
From: Mathieu Desnoyers @ 2019-05-08 14:39 UTC (permalink / raw
To: simon.marchi, jgalar, joraj; +Cc: lttng-dev
bitfield.h uses the left shift operator with a left operand which
may be negative. The C99 standard states that shifting a negative
value is undefined.
When building with -Wshift-negative-value, we get this gcc warning:
In file included from /home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h:44:0,
from /home/smarchi/src/babeltrace/ctfser/ctfser.c:42:
/home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h: In function ‘bt_ctfser_write_unsigned_int’:
/home/smarchi/src/babeltrace/include/babeltrace/bitfield-internal.h:116:24: error: left shift of negative value [-Werror=shift-negative-value]
mask = ~((~(type) 0) << (__start % ts)); \
^
/home/smarchi/src/babeltrace/include/babeltrace/bitfield-internal.h:222:2: note: in expansion of macro ‘_bt_bitfield_write_le’
_bt_bitfield_write_le(ptr, type, _start, _length, _v)
^~~~~~~~~~~~~~~~~~~~~
/home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h:418:3: note: in expansion of macro ‘bt_bitfield_write_le’
bt_bitfield_write_le(mmap_align_addr(ctfser->base_mma) +
^~~~~~~~~~~~~~~~~~~~
This boils down to the fact that the expression ~((uint8_t)0) has type
"signed int", which is used as an operand of the left shift. This is due
to the integer promotion rules of C99 (6.3.3.1):
If an int can represent all values of the original type, the value is
converted to an int; otherwise, it is converted to an unsigned int.
These are called the integer promotions. All other types are unchanged
by the integer promotions.
We also need to cast the result explicitly into the left hand
side type to deal with:
warning: large integer implicitly truncated to unsigned type [-Woverflow]
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
Changes since v1:
- Generate compile-time error if the type argument passed to
_bt_unsigned_cast() is larger than sizeof(uint64_t), this
allows removing _bt_check_max_64bit,
- Introduce _br_fill_mask, which replaces _bt_bitwise_not,
- Clarify _bt_unsigned_cast comment,
- Expand explanation of the issue within the patch commit message.
---
include/babeltrace/bitfield-internal.h | 77 ++++++++++++++++------------------
1 file changed, 37 insertions(+), 40 deletions(-)
diff --git a/include/babeltrace/bitfield-internal.h b/include/babeltrace/bitfield-internal.h
index c5d5eccd..7eb36989 100644
--- a/include/babeltrace/bitfield-internal.h
+++ b/include/babeltrace/bitfield-internal.h
@@ -55,21 +55,26 @@
#define _bt_is_signed_type(type) ((type) -1 < (type) 0)
/*
- * NOTE: The cast to (uint64_t) below ensures that we're not casting a
- * negative value, which is undefined in C. However, this limits the
- * maximum type size of `type` and `v` to 64-bit. The
- * _bt_check_max_64bit() is used to check that the users of this header
- * do not use types with a size greater than 64-bit.
+ * Cast value `v` to an unsigned integer type of the size of type `type`.
+ *
+ * The unsigned cast ensures that we're not shifting a negative value,
+ * which is undefined in C. However, this limits the maximum type size
+ * of `type` to 64-bit. Generate a compile-time error if the size of
+ * `type` is larger than 64-bit.
*/
#define _bt_unsigned_cast(type, v) \
-({ \
- (sizeof(v) < sizeof(type)) ? \
- ((type) (v)) & ((type) (~(~(uint64_t) 0 << (sizeof(v) * CHAR_BIT)))) : \
- (type) (v); \
-})
+ (sizeof(type) == sizeof(uint8_t) ? (uint8_t) (v) : \
+ sizeof(type) == sizeof(uint16_t) ? (uint16_t) (v) : \
+ sizeof(type) == sizeof(uint32_t) ? (uint32_t) (v) : \
+ sizeof(type) == sizeof(uint64_t) ? (uint64_t) (v) : \
+ sizeof(struct { int f:(sizeof(type) > sizeof(uint64_t) ? -1 : 1); }))
-#define _bt_check_max_64bit(type) \
- char _max_64bit_assertion[sizeof(type) <= sizeof(uint64_t) ? 1 : -1] __attribute__((unused))
+/*
+ * _bt_fill_mask evaluates to an unsigned integer with the size of
+ * "type" with all bits set. It is meant to be used as a left operand to
+ * the shift-left operator to create bit masks.
+ */
+#define _bt_fill_mask(type) _bt_unsigned_cast(type, ~(type) 0)
/*
* bt_bitfield_write - write integer to a bitfield in native endianness
@@ -108,15 +113,15 @@ do { \
\
/* Trim v high bits */ \
if (__length < sizeof(__v) * CHAR_BIT) \
- __v &= ~((~(typeof(__v)) 0) << __length); \
+ __v &= (typeof(__v)) ~(_bt_fill_mask(typeof(__v)) << __length); \
\
/* We can now append v with a simple "or", shift it piece-wise */ \
this_unit = start_unit; \
if (start_unit == end_unit - 1) { \
- mask = ~((~(type) 0) << (__start % ts)); \
+ mask = (type) ~(_bt_fill_mask(type) << (__start % ts)); \
if (end % ts) \
- mask |= (~(type) 0) << (end % ts); \
- cmask = (type) __v << (__start % ts); \
+ mask |= (type) (_bt_fill_mask(type) << (end % ts)); \
+ cmask = (type) (_bt_unsigned_cast(type, __v) << (__start % ts)); \
cmask &= ~mask; \
__ptr[this_unit] &= mask; \
__ptr[this_unit] |= cmask; \
@@ -124,8 +129,8 @@ do { \
} \
if (__start % ts) { \
cshift = __start % ts; \
- mask = ~((~(type) 0) << cshift); \
- cmask = (type) __v << cshift; \
+ mask = (type) ~(_bt_fill_mask(type) << cshift); \
+ cmask = (type) (_bt_unsigned_cast(type, __v) << cshift); \
cmask &= ~mask; \
__ptr[this_unit] &= mask; \
__ptr[this_unit] |= cmask; \
@@ -139,7 +144,7 @@ do { \
__start += ts; \
} \
if (end % ts) { \
- mask = (~(type) 0) << (end % ts); \
+ mask = (type) (_bt_fill_mask(type) << (end % ts)); \
cmask = (type) __v; \
cmask &= ~mask; \
__ptr[this_unit] &= mask; \
@@ -167,15 +172,15 @@ do { \
\
/* Trim v high bits */ \
if (__length < sizeof(__v) * CHAR_BIT) \
- __v &= ~((~(typeof(__v)) 0) << __length); \
+ __v &= (typeof(__v)) ~(_bt_fill_mask(typeof(__v)) << __length); \
\
/* We can now append v with a simple "or", shift it piece-wise */ \
this_unit = end_unit - 1; \
if (start_unit == end_unit - 1) { \
- mask = ~((~(type) 0) << ((ts - (end % ts)) % ts)); \
+ mask = (type) ~(_bt_fill_mask(type) << ((ts - (end % ts)) % ts)); \
if (__start % ts) \
- mask |= (~((type) 0)) << (ts - (__start % ts)); \
- cmask = (type) __v << ((ts - (end % ts)) % ts); \
+ mask |= (type) (_bt_fill_mask(type) << (ts - (__start % ts))); \
+ cmask = (type) (_bt_unsigned_cast(type, __v) << ((ts - (end % ts)) % ts)); \
cmask &= ~mask; \
__ptr[this_unit] &= mask; \
__ptr[this_unit] |= cmask; \
@@ -183,8 +188,8 @@ do { \
} \
if (end % ts) { \
cshift = end % ts; \
- mask = ~((~(type) 0) << (ts - cshift)); \
- cmask = (type) __v << (ts - cshift); \
+ mask = (type) ~(_bt_fill_mask(type) << (ts - cshift)); \
+ cmask = (type) (_bt_unsigned_cast(type, __v) << (ts - cshift)); \
cmask &= ~mask; \
__ptr[this_unit] &= mask; \
__ptr[this_unit] |= cmask; \
@@ -198,7 +203,7 @@ do { \
end -= ts; \
} \
if (__start % ts) { \
- mask = (~(type) 0) << (ts - (__start % ts)); \
+ mask = (type) (_bt_fill_mask(type) << (ts - (__start % ts))); \
cmask = (type) __v; \
cmask &= ~mask; \
__ptr[this_unit] &= mask; \
@@ -252,10 +257,6 @@ do { \
unsigned long start_unit, end_unit, this_unit; \
unsigned long end, cshift; /* cshift is "complement shift" */ \
\
- { _bt_check_max_64bit(type); } \
- { _bt_check_max_64bit(typeof(*_vptr)); } \
- { _bt_check_max_64bit(typeof(*_ptr)); } \
- \
if (!__length) { \
*__vptr = 0; \
break; \
@@ -275,7 +276,7 @@ do { \
cmask = __ptr[this_unit]; \
cmask >>= (__start % ts); \
if ((end - __start) % ts) { \
- mask = ~((~(type) 0) << (end - __start)); \
+ mask = (type) ~(_bt_fill_mask(type) << (end - __start)); \
cmask &= mask; \
} \
__v = _bt_piecewise_lshift(__v, end - __start); \
@@ -285,7 +286,7 @@ do { \
} \
if (end % ts) { \
cshift = end % ts; \
- mask = ~((~(type) 0) << cshift); \
+ mask = (type) ~(_bt_fill_mask(type) << cshift); \
cmask = __ptr[this_unit]; \
cmask &= mask; \
__v = _bt_piecewise_lshift(__v, cshift); \
@@ -299,7 +300,7 @@ do { \
end -= ts; \
} \
if (__start % ts) { \
- mask = ~((~(type) 0) << (ts - (__start % ts))); \
+ mask = (type) ~(_bt_fill_mask(type) << (ts - (__start % ts))); \
cmask = __ptr[this_unit]; \
cmask >>= (__start % ts); \
cmask &= mask; \
@@ -323,10 +324,6 @@ do { \
unsigned long start_unit, end_unit, this_unit; \
unsigned long end, cshift; /* cshift is "complement shift" */ \
\
- { _bt_check_max_64bit(type); } \
- { _bt_check_max_64bit(typeof(*_vptr)); } \
- { _bt_check_max_64bit(typeof(*_ptr)); } \
- \
if (!__length) { \
*__vptr = 0; \
break; \
@@ -346,7 +343,7 @@ do { \
cmask = __ptr[this_unit]; \
cmask >>= (ts - (end % ts)) % ts; \
if ((end - __start) % ts) { \
- mask = ~((~(type) 0) << (end - __start)); \
+ mask = (type) ~(_bt_fill_mask(type) << (end - __start)); \
cmask &= mask; \
} \
__v = _bt_piecewise_lshift(__v, end - __start); \
@@ -356,7 +353,7 @@ do { \
} \
if (__start % ts) { \
cshift = __start % ts; \
- mask = ~((~(type) 0) << (ts - cshift)); \
+ mask = (type) ~(_bt_fill_mask(type) << (ts - cshift)); \
cmask = __ptr[this_unit]; \
cmask &= mask; \
__v = _bt_piecewise_lshift(__v, ts - cshift); \
@@ -370,7 +367,7 @@ do { \
__start += ts; \
} \
if (end % ts) { \
- mask = ~((~(type) 0) << (end % ts)); \
+ mask = (type) ~(_bt_fill_mask(type) << (end % ts)); \
cmask = __ptr[this_unit]; \
cmask >>= ts - (end % ts); \
cmask &= mask; \
--
2.11.0
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^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH babeltrace v2] Fix: bitfield: left shift undefined behavior
[not found] <20190508143901.1393-1-mathieu.desnoyers@efficios.com>
@ 2019-05-08 15:49 ` Simon Marchi
[not found] ` <9020b4de-9c66-680f-f28f-6681ebae44c6@simark.ca>
1 sibling, 0 replies; 5+ messages in thread
From: Simon Marchi @ 2019-05-08 15:49 UTC (permalink / raw
To: Mathieu Desnoyers, simon.marchi, jgalar, joraj; +Cc: lttng-dev
On 2019-05-08 10:39 a.m., Mathieu Desnoyers wrote:
> bitfield.h uses the left shift operator with a left operand which
> may be negative. The C99 standard states that shifting a negative
> value is undefined.
>
> When building with -Wshift-negative-value, we get this gcc warning:
>
> In file included from /home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h:44:0,
> from /home/smarchi/src/babeltrace/ctfser/ctfser.c:42:
> /home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h: In function ‘bt_ctfser_write_unsigned_int’:
> /home/smarchi/src/babeltrace/include/babeltrace/bitfield-internal.h:116:24: error: left shift of negative value [-Werror=shift-negative-value]
> mask = ~((~(type) 0) << (__start % ts)); \
> ^
> /home/smarchi/src/babeltrace/include/babeltrace/bitfield-internal.h:222:2: note: in expansion of macro ‘_bt_bitfield_write_le’
> _bt_bitfield_write_le(ptr, type, _start, _length, _v)
> ^~~~~~~~~~~~~~~~~~~~~
> /home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h:418:3: note: in expansion of macro ‘bt_bitfield_write_le’
> bt_bitfield_write_le(mmap_align_addr(ctfser->base_mma) +
> ^~~~~~~~~~~~~~~~~~~~
>
> This boils down to the fact that the expression ~((uint8_t)0) has type
> "signed int", which is used as an operand of the left shift. This is due
> to the integer promotion rules of C99 (6.3.3.1):
>
> If an int can represent all values of the original type, the value is
> converted to an int; otherwise, it is converted to an unsigned int.
> These are called the integer promotions. All other types are unchanged
> by the integer promotions.
>
> We also need to cast the result explicitly into the left hand
> side type to deal with:
>
> warning: large integer implicitly truncated to unsigned type [-Woverflow]
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> ---
> Changes since v1:
> - Generate compile-time error if the type argument passed to
> _bt_unsigned_cast() is larger than sizeof(uint64_t), this
> allows removing _bt_check_max_64bit,
> - Introduce _br_fill_mask, which replaces _bt_bitwise_not,
> - Clarify _bt_unsigned_cast comment,
> - Expand explanation of the issue within the patch commit message.
I didn't spot any mistake by inspecting the code, but I get some errors running
tests/lib/test_bitfield, so I guess there are some :). Does it pass on your side?
> ---
> include/babeltrace/bitfield-internal.h | 77 ++++++++++++++++------------------
> 1 file changed, 37 insertions(+), 40 deletions(-)
>
> diff --git a/include/babeltrace/bitfield-internal.h b/include/babeltrace/bitfield-internal.h
> index c5d5eccd..7eb36989 100644
> --- a/include/babeltrace/bitfield-internal.h
> +++ b/include/babeltrace/bitfield-internal.h
> @@ -55,21 +55,26 @@
> #define _bt_is_signed_type(type) ((type) -1 < (type) 0)
>
> /*
> - * NOTE: The cast to (uint64_t) below ensures that we're not casting a
> - * negative value, which is undefined in C. However, this limits the
> - * maximum type size of `type` and `v` to 64-bit. The
> - * _bt_check_max_64bit() is used to check that the users of this header
> - * do not use types with a size greater than 64-bit.
> + * Cast value `v` to an unsigned integer type of the size of type `type`.
> + *
> + * The unsigned cast ensures that we're not shifting a negative value,
> + * which is undefined in C. However, this limits the maximum type size
> + * of `type` to 64-bit. Generate a compile-time error if the size of
> + * `type` is larger than 64-bit.
> */
> #define _bt_unsigned_cast(type, v) \
> -({ \
> - (sizeof(v) < sizeof(type)) ? \
> - ((type) (v)) & ((type) (~(~(uint64_t) 0 << (sizeof(v) * CHAR_BIT)))) : \
> - (type) (v); \
> -})
> + (sizeof(type) == sizeof(uint8_t) ? (uint8_t) (v) : \
> + sizeof(type) == sizeof(uint16_t) ? (uint16_t) (v) : \
> + sizeof(type) == sizeof(uint32_t) ? (uint32_t) (v) : \
> + sizeof(type) == sizeof(uint64_t) ? (uint64_t) (v) : \
> + sizeof(struct { int f:(sizeof(type) > sizeof(uint64_t) ? -1 : 1); }))
>
> -#define _bt_check_max_64bit(type) \
> - char _max_64bit_assertion[sizeof(type) <= sizeof(uint64_t) ? 1 : -1] __attribute__((unused))
> +/*
> + * _bt_fill_mask evaluates to an unsigned integer with the size of
> + * "type" with all bits set. It is meant to be used as a left operand to
> + * the shift-left operator to create bit masks.
> + */
> +#define _bt_fill_mask(type) _bt_unsigned_cast(type, ~(type) 0)
Thanks for the comment, it makes what this macro does very clear.
>
> /*
> * bt_bitfield_write - write integer to a bitfield in native endianness
> @@ -108,15 +113,15 @@ do { \
> \
> /* Trim v high bits */ \
> if (__length < sizeof(__v) * CHAR_BIT) \
> - __v &= ~((~(typeof(__v)) 0) << __length); \
> + __v &= (typeof(__v)) ~(_bt_fill_mask(typeof(__v)) << __length); \
Not a blocker, but this operation is done enough times that it might be worth
giving it a name and make a macro for it. It could help make this big macro
more readable, and we could test it on its own if needed.
/* Generate a mask of type `type` with the `length` least significant bits set. */
#define _bt_make_mask(type, length) \
((type) ~(_bt_fill_mask(type) << length))
Simon
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^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH babeltrace v2] Fix: bitfield: left shift undefined behavior
[not found] ` <9020b4de-9c66-680f-f28f-6681ebae44c6@simark.ca>
@ 2019-05-08 15:59 ` Mathieu Desnoyers
[not found] ` <634308112.1933.1557331191087.JavaMail.zimbra@efficios.com>
1 sibling, 0 replies; 5+ messages in thread
From: Mathieu Desnoyers @ 2019-05-08 15:59 UTC (permalink / raw
To: Simon Marchi
Cc: Simon Marchi, Jeremie Galarneau, lttng-dev,
Jonathan Rajotte-Julien
----- On May 8, 2019, at 11:49 AM, Simon Marchi simark@simark.ca wrote:
> On 2019-05-08 10:39 a.m., Mathieu Desnoyers wrote:
>> bitfield.h uses the left shift operator with a left operand which
>> may be negative. The C99 standard states that shifting a negative
>> value is undefined.
>>
>> When building with -Wshift-negative-value, we get this gcc warning:
>>
>> In file included from
>> /home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h:44:0,
>> from /home/smarchi/src/babeltrace/ctfser/ctfser.c:42:
>> /home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h: In function
>> ‘bt_ctfser_write_unsigned_int’:
>> /home/smarchi/src/babeltrace/include/babeltrace/bitfield-internal.h:116:24:
>> error: left shift of negative value [-Werror=shift-negative-value]
>> mask = ~((~(type) 0) << (__start % ts)); \
>> ^
>> /home/smarchi/src/babeltrace/include/babeltrace/bitfield-internal.h:222:2: note:
>> in expansion of macro ‘_bt_bitfield_write_le’
>> _bt_bitfield_write_le(ptr, type, _start, _length, _v)
>> ^~~~~~~~~~~~~~~~~~~~~
>> /home/smarchi/src/babeltrace/include/babeltrace/ctfser-internal.h:418:3: note:
>> in expansion of macro ‘bt_bitfield_write_le’
>> bt_bitfield_write_le(mmap_align_addr(ctfser->base_mma) +
>> ^~~~~~~~~~~~~~~~~~~~
>>
>> This boils down to the fact that the expression ~((uint8_t)0) has type
>> "signed int", which is used as an operand of the left shift. This is due
>> to the integer promotion rules of C99 (6.3.3.1):
>>
>> If an int can represent all values of the original type, the value is
>> converted to an int; otherwise, it is converted to an unsigned int.
>> These are called the integer promotions. All other types are unchanged
>> by the integer promotions.
>>
>> We also need to cast the result explicitly into the left hand
>> side type to deal with:
>>
>> warning: large integer implicitly truncated to unsigned type [-Woverflow]
>>
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> ---
>> Changes since v1:
>> - Generate compile-time error if the type argument passed to
>> _bt_unsigned_cast() is larger than sizeof(uint64_t), this
>> allows removing _bt_check_max_64bit,
>> - Introduce _br_fill_mask, which replaces _bt_bitwise_not,
>> - Clarify _bt_unsigned_cast comment,
>> - Expand explanation of the issue within the patch commit message.
>
> I didn't spot any mistake by inspecting the code, but I get some errors running
> tests/lib/test_bitfield, so I guess there are some :). Does it pass on your
> side?
What compiler do you use, and which compilation flags ?
(it works here)
[...]
>>
>> /*
>> * bt_bitfield_write - write integer to a bitfield in native endianness
>> @@ -108,15 +113,15 @@ do { \
>> \
>> /* Trim v high bits */ \
>> if (__length < sizeof(__v) * CHAR_BIT) \
>> - __v &= ~((~(typeof(__v)) 0) << __length); \
>> + __v &= (typeof(__v)) ~(_bt_fill_mask(typeof(__v)) << __length); \
>
> Not a blocker, but this operation is done enough times that it might be worth
> giving it a name and make a macro for it. It could help make this big macro
> more readable, and we could test it on its own if needed.
>
> /* Generate a mask of type `type` with the `length` least significant bits set.
> */
>
> #define _bt_make_mask(type, length) \
> ((type) ~(_bt_fill_mask(type) << length))
Good point! Will do. Missing () around "length" though.
Thanks,
Mathieu
>
> Simon
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH babeltrace v2] Fix: bitfield: left shift undefined behavior
[not found] ` <634308112.1933.1557331191087.JavaMail.zimbra@efficios.com>
@ 2019-05-08 16:08 ` Simon Marchi
[not found] ` <23cd90d8-daaf-9951-c183-9315e6d7db96@efficios.com>
1 sibling, 0 replies; 5+ messages in thread
From: Simon Marchi @ 2019-05-08 16:08 UTC (permalink / raw
To: Mathieu Desnoyers, Simon Marchi
Cc: lttng-dev, Jeremie Galarneau, Jonathan Rajotte-Julien
On 2019-05-08 11:59 a.m., Mathieu Desnoyers wrote:
> What compiler do you use, and which compilation flags ?
> (it works here)
"gcc (Ubuntu 7.4.0-1ubuntu1~18.04) 7.4.0", which is the system compiler on Ubuntu 18.04.
I just built with
./configure 'CFLAGS=-g3 -O0 -fsanitize=address -Wall'
and got some failures, the first one being
not ok 8 - Writing and reading back 0xC07EBC7, signed
# Failed test (test_bitfield.c:run_test_signed() at line 224)
# Failed reading value written "signed char"-wise, with start=0 and length=29. Read FFFFFFFFFFFFFFC7
# 0xFFFFFFC7 0xFFFFFFEB 0x7 0xC 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
The test passes without the patch. If you can't reproduce it easily, I can dig
on my side.
>>>
>>> /*
>>> * bt_bitfield_write - write integer to a bitfield in native endianness
>>> @@ -108,15 +113,15 @@ do { \
>>> \
>>> /* Trim v high bits */ \
>>> if (__length < sizeof(__v) * CHAR_BIT) \
>>> - __v &= ~((~(typeof(__v)) 0) << __length); \
>>> + __v &= (typeof(__v)) ~(_bt_fill_mask(typeof(__v)) << __length); \
>>
>> Not a blocker, but this operation is done enough times that it might be worth
>> giving it a name and make a macro for it. It could help make this big macro
>> more readable, and we could test it on its own if needed.
>>
>> /* Generate a mask of type `type` with the `length` least significant bits set.
>> */
>>
>> #define _bt_make_mask(type, length) \
>> ((type) ~(_bt_fill_mask(type) << length))
>
> Good point! Will do. Missing () around "length" though.
Right, and I should have mentioned I didn't actually test that macro, so handle with care!
Simon
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH babeltrace v2] Fix: bitfield: left shift undefined behavior
[not found] ` <23cd90d8-daaf-9951-c183-9315e6d7db96@efficios.com>
@ 2019-05-08 17:01 ` Mathieu Desnoyers
0 siblings, 0 replies; 5+ messages in thread
From: Mathieu Desnoyers @ 2019-05-08 17:01 UTC (permalink / raw
To: Simon Marchi
Cc: Simon Marchi, lttng-dev, Jeremie Galarneau,
Jonathan Rajotte-Julien
----- On May 8, 2019, at 12:08 PM, Simon Marchi simon.marchi@efficios.com wrote:
> On 2019-05-08 11:59 a.m., Mathieu Desnoyers wrote:
>> What compiler do you use, and which compilation flags ?
>> (it works here)
>
> "gcc (Ubuntu 7.4.0-1ubuntu1~18.04) 7.4.0", which is the system compiler on
> Ubuntu 18.04.
>
> I just built with
>
> ./configure 'CFLAGS=-g3 -O0 -fsanitize=address -Wall'
>
> and got some failures, the first one being
>
> not ok 8 - Writing and reading back 0xC07EBC7, signed
> # Failed test (test_bitfield.c:run_test_signed() at line 224)
> # Failed reading value written "signed char"-wise, with start=0 and length=29.
> Read FFFFFFFFFFFFFFC7
> # 0xFFFFFFC7 0xFFFFFFEB 0x7 0xC 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
> 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
> 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
> 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
> 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
> 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
> 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
>
> The test passes without the patch. If you can't reproduce it easily, I can dig
> on my side.
it fails here too... the executable test-bitfield was renamed to test_bitfield
and I had an old tree with the prior executable. Will investigate.
Thanks,
Mathieu
>
>>>>
>>>> /*
>>>> * bt_bitfield_write - write integer to a bitfield in native endianness
>>>> @@ -108,15 +113,15 @@ do { \
>>>> \
>>>> /* Trim v high bits */ \
>>>> if (__length < sizeof(__v) * CHAR_BIT) \
>>>> - __v &= ~((~(typeof(__v)) 0) << __length); \
>>>> + __v &= (typeof(__v)) ~(_bt_fill_mask(typeof(__v)) << __length); \
>>>
>>> Not a blocker, but this operation is done enough times that it might be worth
>>> giving it a name and make a macro for it. It could help make this big macro
>>> more readable, and we could test it on its own if needed.
>>>
>>> /* Generate a mask of type `type` with the `length` least significant bits set.
>>> */
>>>
>>> #define _bt_make_mask(type, length) \
>>> ((type) ~(_bt_fill_mask(type) << length))
>>
>> Good point! Will do. Missing () around "length" though.
>
> Right, and I should have mentioned I didn't actually test that macro, so handle
> with care!
>
> Simon
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2019-05-08 17:01 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz follow: Atom feed
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[not found] <20190508143901.1393-1-mathieu.desnoyers@efficios.com>
2019-05-08 15:49 ` [PATCH babeltrace v2] Fix: bitfield: left shift undefined behavior Simon Marchi
[not found] ` <9020b4de-9c66-680f-f28f-6681ebae44c6@simark.ca>
2019-05-08 15:59 ` Mathieu Desnoyers
[not found] ` <634308112.1933.1557331191087.JavaMail.zimbra@efficios.com>
2019-05-08 16:08 ` Simon Marchi
[not found] ` <23cd90d8-daaf-9951-c183-9315e6d7db96@efficios.com>
2019-05-08 17:01 ` Mathieu Desnoyers
2019-05-08 14:39 Mathieu Desnoyers
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