bpf: Report arena faults to BPF stderr

Begin reporting arena page faults and the faulting address to BPF
program's stderr, this patch adds support in the arm64 and x86-64 JITs,
support for other archs can be added later.

The fault handlers receive the 32 bit address in the arena region so
the upper 32 bits of user_vm_start is added to it before printing the
address. This is what the user would expect to see as this is what is
printed by bpf_printk() is you pass it an address returned by
bpf_arena_alloc_pages();

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250911145808.58042-4-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 80 insertions, 5 deletions

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 7e3fca164620..8d34a9400a5e 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -8,6 +8,7 @@
 #include <linux/netdevice.h>
 #include <linux/filter.h>
 #include <linux/if_vlan.h>
+#include <linux/bitfield.h>
 #include <linux/bpf.h>
 #include <linux/memory.h>
 #include <linux/sort.h>
@@ -1388,16 +1389,67 @@ static int emit_atomic_ld_st_index(u8 **pprog, u32 atomic_op, u32 size,
 	return 0;
 }
 
+/*
+ * Metadata encoding for exception handling in JITed code.
+ *
+ * Format of `fixup` and `data` fields in `struct exception_table_entry`:
+ *
+ * Bit layout of `fixup` (32-bit):
+ *
+ * +-----------+--------+-----------+---------+----------+
+ * | 31        | 30-24  |   23-16   |   15-8  |    7-0   |
+ * |           |        |           |         |          |
+ * | ARENA_ACC | Unused | ARENA_REG | DST_REG | INSN_LEN |
+ * +-----------+--------+-----------+---------+----------+
+ *
+ * - INSN_LEN (8 bits): Length of faulting insn (max x86 insn = 15 bytes (fits in 8 bits)).
+ * - DST_REG  (8 bits): Offset of dst_reg from reg2pt_regs[] (max offset = 112 (fits in 8 bits)).
+ *                      This is set to DONT_CLEAR if the insn is a store.
+ * - ARENA_REG (8 bits): Offset of the register that is used to calculate the
+ *                       address for load/store when accessing the arena region.
+ * - ARENA_ACCESS (1 bit): This bit is set when the faulting instruction accessed the arena region.
+ *
+ * Bit layout of `data` (32-bit):
+ *
+ * +--------------+--------+--------------+
+ * |	31-16	  |  15-8  |     7-0      |
+ * |              |	   |              |
+ * | ARENA_OFFSET | Unused |  EX_TYPE_BPF |
+ * +--------------+--------+--------------+
+ *
+ * - ARENA_OFFSET (16 bits): Offset used to calculate the address for load/store when
+ *                           accessing the arena region.
+ */
+
 #define DONT_CLEAR 1
+#define FIXUP_INSN_LEN_MASK	GENMASK(7, 0)
+#define FIXUP_REG_MASK		GENMASK(15, 8)
+#define FIXUP_ARENA_REG_MASK	GENMASK(23, 16)
+#define FIXUP_ARENA_ACCESS	BIT(31)
+#define DATA_ARENA_OFFSET_MASK	GENMASK(31, 16)
 
 bool ex_handler_bpf(const struct exception_table_entry *x, struct pt_regs *regs)
 {
-	u32 reg = x->fixup >> 8;
+	u32 reg = FIELD_GET(FIXUP_REG_MASK, x->fixup);
+	u32 insn_len = FIELD_GET(FIXUP_INSN_LEN_MASK, x->fixup);
+	bool is_arena = !!(x->fixup & FIXUP_ARENA_ACCESS);
+	bool is_write = (reg == DONT_CLEAR);
+	unsigned long addr;
+	s16 off;
+	u32 arena_reg;
+
+	if (is_arena) {
+		arena_reg = FIELD_GET(FIXUP_ARENA_REG_MASK, x->fixup);
+		off = FIELD_GET(DATA_ARENA_OFFSET_MASK, x->data);
+		addr = *(unsigned long *)((void *)regs + arena_reg) + off;
+		bpf_prog_report_arena_violation(is_write, addr, regs->ip);
+	}
 
 	/* jump over faulting load and clear dest register */
 	if (reg != DONT_CLEAR)
 		*(unsigned long *)((void *)regs + reg) = 0;
-	regs->ip += x->fixup & 0xff;
+	regs->ip += insn_len;
+
 	return true;
 }
 
@@ -2070,6 +2122,7 @@ populate_extable:
 			{
 				struct exception_table_entry *ex;
 				u8 *_insn = image + proglen + (start_of_ldx - temp);
+				u32 arena_reg, fixup_reg;
 				s64 delta;
 
 				if (!bpf_prog->aux->extable)
@@ -2089,8 +2142,29 @@ populate_extable:
 
 				ex->data = EX_TYPE_BPF;
 
-				ex->fixup = (prog - start_of_ldx) |
-					((BPF_CLASS(insn->code) == BPF_LDX ? reg2pt_regs[dst_reg] : DONT_CLEAR) << 8);
+				/*
+				 * src_reg/dst_reg holds the address in the arena region with upper
+				 * 32-bits being zero because of a preceding addr_space_cast(r<n>,
+				 * 0x0, 0x1) instruction. This address is adjusted with the addition
+				 * of arena_vm_start (see the implementation of BPF_PROBE_MEM32 and
+				 * BPF_PROBE_ATOMIC) before being used for the memory access. Pass
+				 * the reg holding the unmodified 32-bit address to
+				 * ex_handler_bpf().
+				 */
+				if (BPF_CLASS(insn->code) == BPF_LDX) {
+					arena_reg = reg2pt_regs[src_reg];
+					fixup_reg = reg2pt_regs[dst_reg];
+				} else {
+					arena_reg = reg2pt_regs[dst_reg];
+					fixup_reg = DONT_CLEAR;
+				}
+
+				ex->fixup = FIELD_PREP(FIXUP_INSN_LEN_MASK, prog - start_of_ldx) |
+					    FIELD_PREP(FIXUP_ARENA_REG_MASK, arena_reg) |
+					    FIELD_PREP(FIXUP_REG_MASK, fixup_reg);
+				ex->fixup |= FIXUP_ARENA_ACCESS;
+
+				ex->data |= FIELD_PREP(DATA_ARENA_OFFSET_MASK, insn->off);
 			}
 			break;
 
@@ -2208,7 +2282,8 @@ populate_extable:
 				 * End result: x86 insn "mov rbx, qword ptr [rax+0x14]"
 				 * of 4 bytes will be ignored and rbx will be zero inited.
 				 */
-				ex->fixup = (prog - start_of_ldx) | (reg2pt_regs[dst_reg] << 8);
+				ex->fixup = FIELD_PREP(FIXUP_INSN_LEN_MASK, prog - start_of_ldx) |
+					    FIELD_PREP(FIXUP_REG_MASK, reg2pt_regs[dst_reg]);
 			}
 			break;