24 Jul
2021
24 Jul
'21
4:24 p.m.
Sparse will try to check casting of simple integer types which are marked
as __bitwise. This for example "disallows" simple casting of __be{16,32,64}
or __le{16,32,64} to other types. This is also true for pointers to
variables with this type.
But the new generic {get,put}_unaligned is doing that by (reinterpret)
casting the original pointer to a new (anonymous) struct pointer. This will
then create warnings like:
net/batman-adv/distributed-arp-table.c:1461:19: warning: cast from restricted __be32 *
net/batman-adv/distributed-arp-table.c:1510:23: warning: cast from restricted __be32
[usertype] *[assigned] magic
net/batman-adv/distributed-arp-table.c:1588:24: warning: cast from restricted __be32
[usertype] *[assigned] yiaddr
The special attribute force must be used in such statements when the cast
is known to be safe to avoid these warnings.
Fixes: 803f4e1eab7a ("asm-generic: simplify asm/unaligned.h")
Signed-off-by: Sven Eckelmann <sven(a)narfation.org>
---
include/asm-generic/unaligned.h | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/include/asm-generic/unaligned.h b/include/asm-generic/unaligned.h
index 1c4242416c9f..e2b23e5bf945 100644
--- a/include/asm-generic/unaligned.h
+++ b/include/asm-generic/unaligned.h
@@ -10,12 +10,13 @@
#include <asm/byteorder.h>
#define __get_unaligned_t(type, ptr) ({ \
- const struct { type x; } __packed *__pptr = (typeof(__pptr))(ptr); \
+ const struct { type x; } __packed *__pptr; \
+ __pptr = (__force typeof(__pptr))(ptr); \
__pptr->x; \
})
#define __put_unaligned_t(type, val, ptr) do { \
- struct { type x; } __packed *__pptr = (typeof(__pptr))(ptr); \
+ struct { type x; } __packed *__pptr = (__force typeof(__pptr))(ptr); \
__pptr->x = (val); \
} while (0)
--
2.30.2
24 Jul
24 Jul
5:01 p.m.
On Sat, Jul 24, 2021 at 06:24:29PM +0200, Sven Eckelmann wrote:
The special attribute force must be used in such
statements when the cast
is known to be safe to avoid these warnings.
How about container_of(ptr, typeof(*__pptr), x) instead of a cast?
Would be easier to follow...
26 Jul
26 Jul
12:57 p.m.
On Sat, Jul 24, 2021 at 7:01 PM Al Viro <viro(a)zeniv.linux.org.uk> wrote:
On Sat, Jul 24, 2021 at 06:24:29PM +0200, Sven Eckelmann wrote:
> The special attribute force must be used in such statements when the cast
> is known to be safe to avoid these warnings.
I can see why this would warn, but I'm having trouble reproducing the
warning on linux-next.
How about container_of(ptr, typeof(*__pptr),
x) instead of a cast?
Would be easier to follow...
If both work equally well, I'd prefer Sven's patch since that only
expands 'type'
once, while container_of() expands it three more times. This may not make
much of a difference, but I've seen a number of cases where nested macros
can explode the preprocessed code size enough to slow down kernel compilation
over all, and it's quite possible to have get_unaligned()/put_unaligned in
the middle of that, with a complex expression passed into that.
Arnd
3:04 p.m.
On Monday, 26 July 2021 14:57:31 CEST Arnd Bergmann wrote:
I have sparse 0.6.3 on an Debian bullseye amd64 system. Sources are from
linux-next next-20210723
make allnoconfig
cat >> .config << "EOF"
CONFIG_NET=y
CONFIG_INET=y
CONFIG_BATMAN_ADV=y
CONFIG_BATMAN_ADV_DAT=y
EOF
make olddefconfig
make CHECK="sparse -Wbitwise-pointer" C=1
I should maybe have made this clearer in the last sentence of the first
paragraph: "This is also true for pointers to variables with this type when
-Wbitwise-pointer is activated."
Kind regards,
Sven
> The special attribute force must be used in such statements when the cast
> is known to be safe to avoid these warnings.
I can see why this would warn, but I'm having trouble reproducing the
warning on linux-next.
4:22 p.m.
On Mon, Jul 26, 2021 at 5:04 PM Sven Eckelmann <sven(a)narfation.org> wrote:
On Monday, 26 July 2021 14:57:31 CEST Arnd Bergmann wrote:
I have sparse 0.6.3 on an Debian bullseye amd64 system. Sources are from
linux-next next-20210723
make allnoconfig
cat >> .config << "EOF"
CONFIG_NET=y
CONFIG_INET=y
CONFIG_BATMAN_ADV=y
CONFIG_BATMAN_ADV_DAT=y
EOF
make olddefconfig
make CHECK="sparse -Wbitwise-pointer" C=1
I should maybe have made this clearer in the last sentence of the first
paragraph: "This is also true for pointers to variables with this type when
-Wbitwise-pointer is activated."
Ok, got it. I assumed this would be turned on by an 'allmodconfig' build.
> The special attribute force must be used in such statements when the cast
> is known to be safe to avoid these warnings.
I can see why this would warn, but I'm having trouble reproducing the
warning on linux-next. > If both work equally well, I'd prefer
Sven's patch since that only
> expands 'type' once, while container_of() expands it three more times
Not sure what I was thinking here, as it's not 'type' that gets expanded
here but 'ptr'. We could do Al's suggestion to avoid the __force without
multiple expansions, using
diff --git a/include/asm-generic/unaligned.h b/include/asm-generic/unaligned.h
index 1c4242416c9f..d138dc5fd8e3 100644
--- a/include/asm-generic/unaligned.h
+++ b/include/asm-generic/unaligned.h
@@ -10,17 +10,25 @@
#include <asm/byteorder.h>
#define __get_unaligned_t(type, ptr) ({
\
- const struct { type x; } __packed *__pptr =
(typeof(__pptr))(ptr); \
+ const struct { type x; } __packed *__pptr =
\
+ container_of(ptr, typeof(*__pptr), x);
\
__pptr->x;
\
})
#define __put_unaligned_t(type, val, ptr) do {
\
- struct { type x; } __packed *__pptr = (typeof(__pptr))(ptr);
\
+ struct { type x; } __packed *__pptr =
\
+ container_of(ptr, typeof(*__pptr), x);
\
__pptr->x = (val);
\
} while (0)
-#define get_unaligned(ptr) __get_unaligned_t(typeof(*(ptr)), (ptr))
-#define put_unaligned(val, ptr) __put_unaligned_t(typeof(*(ptr)), (val), (ptr))
+#define get_unaligned(ptr) ({
\
+ __auto_type _ptr = (ptr);
\
+ __get_unaligned_t(typeof(*(_ptr)), (_ptr));
\
+})
+#define put_unaligned(val, ptr) ({
\
+ __auto_type _ptr = (ptr);
\
+ __put_unaligned_t(typeof(*(_ptr)), (val), (_ptr));
\
+})
static inline u16 get_unaligned_le16(const void *p)
{
Not sure if this is any better.
Arnd
27 Jul
27 Jul
8:44 a.m.
From: Sven Eckelmann
Sent: 24 July 2021 17:24
Sparse will try to check casting of simple integer types which are marked
as __bitwise. This for example "disallows" simple casting of __be{16,32,64}
or __le{16,32,64} to other types. This is also true for pointers to
variables with this type.
But the new generic {get,put}_unaligned is doing that by (reinterpret)
casting the original pointer to a new (anonymous) struct pointer. This will
then create warnings like:
net/batman-adv/distributed-arp-table.c:1461:19: warning: cast from restricted __be32 *
net/batman-adv/distributed-arp-table.c:1510:23: warning: cast from restricted __be32
[usertype]
*[assigned] magic
net/batman-adv/distributed-arp-table.c:1588:24: warning: cast from restricted __be32
[usertype]
*[assigned] yiaddr
The special attribute force must be used in such statements when the cast
is known to be safe to avoid these warnings.
At least the __force is being added to an existing cast.
The real problems are when a (__force __le32)value cast is used
to silence sparse.
These should really be something like:
__tell_sparce(__le32, value)
so that the whole thing can be removed by the preprocessor when
compiling the code.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
611
days inactive
614
days old
b.a.t.m.a.n@lists.open-mesh.org
5 comments
4 participants
participants (4)
-
Al Viro -
Arnd Bergmann -
David Laight -
Sven Eckelmann