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bpf: clat: avoid 32-bit register spills when access skb->data

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The verifier reports this error when accessing skb->data:

  ; void *data     = (void *)(unsigned long long)skb->data; @ clat.bpf.c:625
  (61) r2 = *(u32 *)(r1 +76)       ; frame1: R1=ctx() R2_w=pkt(r=0)
  (63) *(u32 *)(r10 -120) = r2
  invalid size of register spill

Apparently it's trying to spill only 32 bits from the register to the
stack, which is invalid. A similar problem was reported here:
https://github.com/cilium/cilium/pull/25336

Add some macros using inline asm to fix the problem. With this change
now the compiler properly generates 64-bit spills.

 ; src/core/bpf/clat.bpf.c:625
-;     void *data     = (void *)(unsigned long long)skb->data;
+;     void *data     = SKB_DATA(skb);
      137:      61 12 4c 00 00 00 00 00 w2 = *(u32 *)(r1 + 0x4c)
-     138:      63 2a 88 ff 00 00 00 00 *(u32 *)(r10 - 0x78) = w2
+     138:      7b 2a 88 ff 00 00 00 00 *(u64 *)(r10 - 0x78) = r2
This commit is contained in:
Beniamino Galvani 2025-10-03 17:48:01 +02:00
parent 9bf54393fc
commit cee2be7d37
1 changed files with 30 additions and 16 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

46
src/core/bpf/clat.bpf.c
View file

@ -61,6 +61,22 @@ struct {
#define DBG(fmt, ...)
#endif
/* Macros to read the sk_buff data* pointers, preventing the compiler
* from generating a 32-bit register spill. */
#define SKB_ACCESS_MEMBER_32(_skb, member) \
({ \
void *ptr; \
\
asm volatile("%0 = *(u32 *)(%1 + %2)" \
: "=r"(ptr) \
: "r"(_skb), "i"(offsetof(struct __sk_buff, member))); \
\
ptr; \
})
#define SKB_DATA(_skb) SKB_ACCESS_MEMBER_32(_skb, data)
#define SKB_DATA_END(_skb) SKB_ACCESS_MEMBER_32(_skb, data_end)
struct icmpv6_pseudo {
struct in6_addr saddr;
struct in6_addr daddr;
@ -76,7 +92,7 @@ update_l4_checksum(struct __sk_buff *skb,
int ip_type,
bool v4to6)
{
void *data = (void *) (unsigned long long) skb->data;
void *data = SKB_DATA(skb);
int flags = BPF_F_PSEUDO_HDR;
__u16 offset;
__u32 csum;
@ -119,7 +135,7 @@ update_icmp_checksum(struct __sk_buff *skb,
void *icmp_after,
bool add)
{
void *data = (void *) (unsigned long long) skb->data;
void *data = SKB_DATA(skb);
struct icmpv6_pseudo ph = {.nh = IPPROTO_ICMPV6,
.saddr = ip6h->saddr,
.daddr = ip6h->daddr,
@ -161,8 +177,7 @@ update_icmp_checksum(struct __sk_buff *skb,
static int
rewrite_icmp(struct iphdr *iph, struct ipv6hdr *ip6h, struct __sk_buff *skb)
{
void *data_end = (void *) (unsigned long long) skb->data_end;
void *data_end = SKB_DATA_END(skb);
struct icmphdr old_icmp, *icmp = (void *) (iph + 1);
struct icmp6hdr icmp6, *new_icmp6;
__u32 mtu;
@ -278,8 +293,8 @@ static __attribute__((always_inline)) inline int
clat_handle_v4(struct __sk_buff *skb, struct hdr_cursor *nh)
{
int ret = TC_ACT_OK;
void *data_end = (void *) (unsigned long long) skb->data_end;
void *data = (void *) (unsigned long long) skb->data;
void *data_end = SKB_DATA_END(skb);
void *data = SKB_DATA(skb);
int ip_type, iphdr_len, ip_offset;
struct in6_addr *dst_v6;
@ -363,8 +378,8 @@ clat_handle_v4(struct __sk_buff *skb, struct hdr_cursor *nh)
if (bpf_skb_change_proto(skb, bpf_htons(ETH_P_IPV6), 0))
goto out;
data = (void *) (unsigned long long) skb->data;
data_end = (void *) (unsigned long long) skb->data_end;
data = SKB_DATA(skb);
data_end = SKB_DATA_END(skb);
eth = data;
ip6h = data + ip_offset;
@ -400,8 +415,7 @@ rewrite_icmpv6(struct ipv6hdr *ip6h,
struct icmphdr **new_icmp_out,
struct hdr_cursor *nh)
{
void *data_end = (void *) (unsigned long long) skb->data_end;
void *data_end = SKB_DATA_END(skb);
struct icmp6hdr old_icmp6, *icmp6 = (void *) (ip6h + 1);
struct icmphdr icmp, *new_icmp;
__u32 mtu, ptr;
@ -631,8 +645,8 @@ static __attribute__((always_inline)) inline int
clat_handle_v6(struct __sk_buff *skb, struct hdr_cursor *nh)
{
int ret = TC_ACT_OK;
void *data_end = (void *) (unsigned long long) skb->data_end;
void *data = (void *) (unsigned long long) skb->data;
void *data_end = SKB_DATA_END(skb);
void *data = SKB_DATA(skb);
struct ethhdr *eth;
struct iphdr *iph;
@ -648,8 +662,8 @@ clat_handle_v6(struct __sk_buff *skb, struct hdr_cursor *nh)
if (bpf_skb_change_proto(skb, bpf_htons(ETH_P_IP), 0))
goto out;
data = (void *) (unsigned long long) skb->data;
data_end = (void *) (unsigned long long) skb->data_end;
data = SKB_DATA(skb);
data_end = SKB_DATA_END(skb);
eth = data;
iph = data + ip_offset;
@ -667,8 +681,8 @@ out:
static __attribute__((always_inline)) inline int
clat_handler(struct __sk_buff *skb, bool egress)
{
void *data_end = (void *) (unsigned long long) skb->data_end;
void *data = (void *) (unsigned long long) skb->data;
void *data_end = SKB_DATA_END(skb);
void *data = SKB_DATA(skb);
struct hdr_cursor nh = {.pos = data};
struct ethhdr *eth;
int eth_type;