[PATCH ipsec-next v2 3/6] libbpf: Add BPF_CORE_WRITE_BITFIELD() macro

[Daniel Xu]

Similar to reading from CO-RE bitfields, we need a CO-RE aware bitfield
writing wrapper to make the verifier happy.

Two alternatives to this approach are:

1. Use the upcoming `preserve_static_offset` [0] attribute to disable
   CO-RE on specific structs.
2. Use broader byte-sized writes to write to bitfields.

(1) is a bit a bit hard to use. It requires specific and
not-very-obvious annotations to bpftool generated vmlinux.h. It's also
not generally available in released LLVM versions yet.

(2) makes the code quite hard to read and write. And especially if
BPF_CORE_READ_BITFIELD() is already being used, it makes more sense to
to have an inverse helper for writing.

[0]: https://reviews.llvm.org/D133361
From: Eduard Zingerman <eddyz87@xxxxxxxxx>

Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
---
 tools/lib/bpf/bpf_core_read.h | 36 +++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
index 1ac57bb7ac55..7a764f65d299 100644
--- a/tools/lib/bpf/bpf_core_read.h
+++ b/tools/lib/bpf/bpf_core_read.h
@@ -111,6 +111,42 @@ enum bpf_enum_value_kind {
 	val;								      \
 })
 
+/*
+ * Write to a bitfield, identified by s->field.
+ * This is the inverse of BPF_CORE_WRITE_BITFIELD().
+ */
+#define BPF_CORE_WRITE_BITFIELD(s, field, new_val) ({			\
+	void *p = (void *)s + __CORE_RELO(s, field, BYTE_OFFSET);	\
+	unsigned int byte_size = __CORE_RELO(s, field, BYTE_SIZE);	\
+	unsigned int lshift = __CORE_RELO(s, field, LSHIFT_U64);	\
+	unsigned int rshift = __CORE_RELO(s, field, RSHIFT_U64);	\
+	unsigned int bit_size = (rshift - lshift);			\
+	unsigned long long nval, val, hi, lo;				\
+									\
+	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;			\
+	}								\
+})
+
 #define ___bpf_field_ref1(field)	(field)
 #define ___bpf_field_ref2(type, field)	(((typeof(type) *)0)->field)
 #define ___bpf_field_ref(args...)					    \
-- 
2.42.1