Re: [PATCH v4 1/7] kcsan: Add atomic builtin stubs for 32-bit systems

[Rohan McLure <rmclure@linux.ibm.com>]

> On 8 Feb 2023, at 11:23 pm, Christophe Leroy <christophe.leroy@csgroup.eu> wrote:
> 
> 
> 
> Le 08/02/2023 à 04:21, Rohan McLure a écrit :
>> KCSAN instruments calls to atomic builtins, and will in turn call these
>> builtins itself. As such, architectures supporting KCSAN must have
>> compiler support for these atomic primitives.
>> 
>> Since 32-bit systems are unlikely to have 64-bit compiler builtins,
>> provide a stub for each missing builtin, and use BUG() to assert
>> unreachability.
>> 
>> In commit 725aea873261 ("xtensa: enable KCSAN"), xtensa implements these
>> locally, but does not advertise the fact with preprocessor macros. To
>> avoid production of duplicate symbols, do not build the stubs on xtensa.
>> A future patch will remove the xtensa implementation of these stubs.
>> 
>> Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
>> ---
>> v4: New patch
>> ---
>>  kernel/kcsan/Makefile |  3 ++
>>  kernel/kcsan/stubs.c  | 78 +++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 81 insertions(+)
>>  create mode 100644 kernel/kcsan/stubs.c
> 
> I think it would be better to merge patch 1 and patch 2, that way we 
> would keep the history and see that stubs.c almost comes from xtensa.
> 
> The summary would then be:
> 
>  arch/xtensa/lib/Makefile                              |  1 -
>  kernel/kcsan/Makefile                                 |  2 +-
>  arch/xtensa/lib/kcsan-stubs.c => kernel/kcsan/stubs.c | 26 
> +++++++++++++++++++++++++-
>  3 files changed, 26 insertions(+), 3 deletions(-)
> 
> 
>> 
>> diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
>> index 8cf70f068d92..5dfc5825aae9 100644
>> --- a/kernel/kcsan/Makefile
>> +++ b/kernel/kcsan/Makefile
>> @@ -12,6 +12,9 @@ CFLAGS_core.o := $(call cc-option,-fno-conserve-stack) \
>>   -fno-stack-protector -DDISABLE_BRANCH_PROFILING
>> 
>>  obj-y := core.o debugfs.o report.o
>> +ifndef XTENSA
>> + obj-y += stubs.o
> 
> obj-$(CONFIG_32BIT) += stubs.o
> 
> That would avoid the #if CONFIG_32BIT in stubs.c

Thanks. Yes happy to do this.

> 
>> +endif
>> 
>>  KCSAN_INSTRUMENT_BARRIERS_selftest.o := y
>>  obj-$(CONFIG_KCSAN_SELFTEST) += selftest.o
>> diff --git a/kernel/kcsan/stubs.c b/kernel/kcsan/stubs.c
>> new file mode 100644
>> index 000000000000..ec5cd99be422
>> --- /dev/null
>> +++ b/kernel/kcsan/stubs.c
>> @@ -0,0 +1,78 @@
>> +// SPDX-License Identifier: GPL-2.0
> 
> Missing - between License and Identifier ?
> 
>> +
>> +#include <linux/bug.h>
>> +#include <linux/types.h>
>> +
>> +#ifdef CONFIG_32BIT
> 
> Should be handled in Makefile
> 
>> +
>> +#if !__has_builtin(__atomic_store_8)
> 
> Does any 32 bit ARCH support that ? Is that #if required ?
> 
> If yes, do we really need the #if for each and every function, can't we 
> just check for one and assume that if we don't have __atomic_store_8 we 
> don't have any of the functions ?

Turns out that testing with gcc provides 8-byte atomic builtins on x86
and arm on 32-bit. However I believe it should just suffice to check for
__atomic_store_8 or any other such builtin i.e. if an arch implements one it
likely implements them all from what I’ve seen.

> 
> 
>> +void __atomic_store_8(volatile void *p, u64 v, int i)
>> +{
>> + BUG();
>> +}
>> +#endif
>> +
>> +#if !__has_builtin(__atomic_load_8)
>> +u64 __atomic_load_8(const volatile void *p, int i)
>> +{
>> + BUG();
>> +}
>> +#endif
>> +
>> +#if !__has_builtin(__atomic_exchange_8)
>> +u64 __atomic_exchange_8(volatile void *p, u64 v, int i)
>> +{
>> + BUG();
>> +}
>> +#endif
>> +
>> +#if !__has_builtin(__atomic_compare_exchange_8)
>> +bool __atomic_compare_exchange_8(volatile void *p1, void *p2, u64 v, bool b, int i1, int i2)
>> +{
>> + BUG();
>> +}
>> +#endif
>> +
>> +#if !__has_builtin(__atomic_fetch_add_8)
>> +u64 __atomic_fetch_add_8(volatile void *p, u64 v, int i)
>> +{
>> + BUG();
>> +}
>> +#endif
>> +
>> +#if !__has_builtin(__atomic_fetch_sub_8)
>> +u64 __atomic_fetch_sub_8(volatile void *p, u64 v, int i)
>> +{
>> + BUG();
>> +}
>> +#endif
>> +
>> +#if !__has_builtin(__atomic_fetch_and_8)
>> +u64 __atomic_fetch_and_8(volatile void *p, u64 v, int i)
>> +{
>> + BUG();
>> +}
>> +#endif
>> +
>> +#if !__has_builtin(__atomic_fetch_or_8)
>> +u64 __atomic_fetch_or_8(volatile void *p, u64 v, int i)
>> +{
>> + BUG();
>> +}
>> +#endif
>> +
>> +#if !__has_builtin(__atomic_fetch_xor_8)
>> +u64 __atomic_fetch_xor_8(volatile void *p, u64 v, int i)
>> +{
>> + BUG();
>> +}
>> +#endif
>> +
>> +#if !__has_builtin(__atomic_fetch_nand_8)
>> +u64 __atomic_fetch_nand_8(volatile void *p, u64 v, int i)
>> +{
>> + BUG();
>> +}
>> +#endif
>> +
>> +#endif /* CONFIG_32BIT */