On 11/26/23 8:52 PM, Eduard Zingerman wrote:
On Sun, 2023-11-26 at 18:04 -0600, Daniel Xu wrote:
[...]
Tbh I'm not sure. This test passes with preserve_static_offset
because it suppresses preserve_access_index. In general clang
translates bitfield access to a set of IR statements like:
C:
struct foo {
unsigned _;
unsigned a:1;
...
};
... foo->a ...
IR:
%a = getelementptr inbounds %struct.foo, ptr %0, i32 0, i32 1
%bf.load = load i8, ptr %a, align 4
%bf.clear = and i8 %bf.load, 1
%bf.cast = zext i8 %bf.clear to i32
With preserve_static_offset the getelementptr+load are replaced by a
single statement which is preserved as-is till code generation,
thus load with align 4 is preserved.
On the other hand, I'm not sure that clang guarantees that load or
stores used for bitfield access would be always aligned according to
verifier expectations.
I think we should check if there are some clang knobs that prevent
generation of unaligned memory access. I'll take a look.
Is there a reason to prefer fixing in compiler? I'm not opposed to it,
but the downside to compiler fix is it takes years to propagate and
sprinkles ifdefs into the code.
Would it be possible to have an analogue of BPF_CORE_READ_BITFIELD()?
Well, the contraption below passes verification, tunnel selftest
appears to work. I might have messed up some shifts in the macro,
though.
I didn't test it. But from high level it should work.
Still, if clang would peek unlucky BYTE_{OFFSET,SIZE} for a particular
field access might be unaligned.
clang should pick a sensible BYTE_SIZE/BYTE_OFFSET to meet
alignment requirement. This is also required for BPF_CORE_READ_BITFIELD.
---
diff --git a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
index 3065a716544d..41cd913ac7ff 100644
--- a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
+++ b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
@@ -9,6 +9,7 @@
#include "vmlinux.h"
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>
+#include <bpf/bpf_core_read.h>
#include "bpf_kfuncs.h"
#include "bpf_tracing_net.h"
@@ -144,6 +145,38 @@ int ip6gretap_get_tunnel(struct __sk_buff *skb)
return TC_ACT_OK;
}
+#define BPF_CORE_WRITE_BITFIELD(s, field, new_val) ({ \
+ void *p = (void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \
+ unsigned byte_size = __CORE_RELO(s, field, BYTE_SIZE); \
+ unsigned lshift = __CORE_RELO(s, field, LSHIFT_U64); \
+ unsigned rshift = __CORE_RELO(s, field, RSHIFT_U64); \
+ unsigned bit_size = (rshift - lshift); \
+ unsigned long long nval, val, hi, lo; \
+ \
+ asm volatile("" : "=r"(p) : "0"(p)); \
Use asm volatile("" : "+r"(p)) ?
+ \
+ switch (byte_size) { \
+ case 1: val = *(unsigned char *)p; break; \
+ case 2: val = *(unsigned short *)p; break; \
+ case 4: val = *(unsigned int *)p; break; \
+ case 8: val = *(unsigned long long *)p; break; \
+ } \
+ hi = val >> (bit_size + rshift); \
+ hi <<= bit_size + rshift; \
+ lo = val << (bit_size + lshift); \
+ lo >>= bit_size + lshift; \
+ nval = new_val; \
+ nval <<= lshift; \
+ nval >>= rshift; \
+ val = hi | nval | lo; \
+ switch (byte_size) { \
+ case 1: *(unsigned char *)p = val; break; \
+ case 2: *(unsigned short *)p = val; break; \
+ case 4: *(unsigned int *)p = val; break; \
+ case 8: *(unsigned long long *)p = val; break; \
+ } \
+})
I think this should be put in libbpf public header files but not sure
where to put it. bpf_core_read.h although it is core write?
But on the other hand, this is a uapi struct bitfield write,
strictly speaking, CORE write is really unnecessary here. It
would be great if we can relieve users from dealing with
such unnecessary CORE writes. In that sense, for this particular
case, I would prefer rewriting the code by using byte-level
stores...
