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@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)
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...
On Sat, Jul 24, 2021 at 7:01 PM Al Viro viro@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
On Monday, 26 July 2021 14:57:31 CEST Arnd Bergmann 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.
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
On Mon, Jul 26, 2021 at 5:04 PM Sven Eckelmann sven@narfation.org wrote:
On Monday, 26 July 2021 14:57:31 CEST Arnd Bergmann 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.
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=1I 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.
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
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
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Al Viro -
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Sven Eckelmann