Sender PID: 40764, Receiver PID: 40761 Sender buf VA: 0x6130204f0069 Receiver buf VA: 0x7ffe943c11d0 Kernel skb dest: 0xffff8cb08de63c00 Kernel skb COPY2: 0xffff8cb00df8acfe
Q0.1: You saw recv_buf PA: 0x1d0 — this is WRONG. Why?
Q0.2: pagemap needs root to read PFN. You ran without root. What did you get?
Q0.3: 0x1d0 = page offset only (0x7ffe943c11d0 & 0xFFF). Calculate yourself:
0x7ffe943c11d0 & 0xFFF = ???
FILL: _____
Q0.4: If PFN were 0x12340, what would full PA be?
PA = (PFN « 12) | page_offset = (0x12340 « 12) | 0x1d0 = ???
FILL: _____
Q0.5: Did pagemap return PFN = 0? What does bit 63 = 0 mean?
Q0.6: How to fix? Run with sudo ./receiver? Or use CAP_SYS_ADMIN?
COUNTER-QUESTION: Why does /proc/self/pagemap hide PFN from non-root? PARADOX: If you can read your own memory (VA), why can’t you see PA?
Q1.1: Kernel skb->data = 0xffff8cb08de63c00. What is PA? Q1.2: Kernel direct map starts at 0xffff888000000000 on your machine. VERIFY: cat /proc/kallsyms | grep “ page_offset_base” RESULT: _____ Q1.3: If page_offset_base = 0xffff888000000000, then: PA = VA - page_offset_base PA = 0xffff8cb08de63c00 - 0xffff888000000000 = ??? FILL: _____ Q1.4: But wait — your VA starts with 0xffff8cb0, not 0xffff8880. Is formula wrong? Q1.5: Check KASLR: cat /proc/cmdline | grep kaslr RESULT: _____ Q1.6: KASLR randomizes kernel base. YOUR machine’s actual page_offset_base is shifted. How to find REAL offset? sudo cat /proc/kallsyms | grep page_offset_base RESULT: _____
COUNTER-QUESTION: If KASLR shifts kernel VA, how does kernel know where RAM is? PARADOX: VA is randomized but PA (RAM) is fixed at boot. Mismatch?
Q2.1: You probed copy_from_iter with dest=0xffff8cb08de63c00. This is skb->data? Q2.2: Look at struct sk_buff in kernel source. Where is ‘data’ field? grep -n “unsigned char.**data” /home/r/Desktop/learn_kernel/source/include/linux/skbuff.h LINE NUMBER: ____ Q2.3: sizeof(struct sk_buff) is ~232 bytes (varies by config). If skb struct is at 0xffff8cb08de63b00, where is skb->data pointer? offset of ‘data’ in sk_buff = ??? (need to count fields) Q2.4: The pointer skb->data POINTS TO buffer. skb->data itself is stored in skb struct. But the VALUE in skb->data points elsewhere. Clarify this. Q2.5: Your kprobe captured dest=0xffff8cb08de63c00. Is this: a) Address of skb->data field in struct? b) Value of skb->data (where payload goes)? ANSWER: _____
COUNTER-QUESTION: kprobe gives regs->di = first argument. For _copy_from_iter(dest, len, iter), dest = where copy WRITES to. So it’s (b). PARADOX: You captured 3 COPY1 entries with same dest but len=22, 1, 37. Why 3 calls?
Real dmesg: [COPY1] PID=40764 comm=sender dest=ffff8cb08de63c00 len=22 [COPY1] PID=40764 comm=sender dest=ffff8cb08de63c00 len=1 [COPY1] PID=40764 comm=sender dest=ffff8cb08de63c00 len=37
Q3.1: You sent 16 bytes “HELLO_SEND_TRACE”. But kernel copied 22 + 1 + 37 = 60 bytes. WHY? Q3.2: UDP packet has: 8 byte UDP header + payload. But 8 + 16 = 24, not 60. Q3.3: IP header is 20 bytes. 20 + 8 + 16 = 44, not 60. Q3.4: Ethernet header is 14 bytes. 14 + 20 + 8 + 16 = 58. Still not 60. Q3.5: Are you probing MORE than just your payload copy? Q3.6: _copy_from_iter is called EVERYWHERE, not just for your send(). Other kernel code uses it. How to FILTER only your 16-byte payload copy?
COUNTER-QUESTION: Your filter is strncmp(current->comm, "sender", 6). But “sender” PID might have other copies happening (DNS, systemd, etc)?
PARADOX: Same dest address 0xffff8cb08de63c00 for all 3 calls. Is kernel reusing same buffer?
Q4.1: You have NO kprobe for receive path. recv_trace_hw.c does not exist. Q4.2: What function does copy_to_user for UDP receive? Candidates: - skb_copy_datagram_iter - skb_copy_datagram_msg - copy_to_iter - copy_to_iter Q4.3: Which is actually called? grep kernel source: grep -rn “copy_to_iter” /home/r/Desktop/learn_kernel/source/net/ipv4/udp.c RESULT: ____ Q4.4: Can you probe copy_to_iter like you probed _copy_from_iter? Check if exported: grep “_copy_to_iter” /proc/kallsyms RESULT: ____ Q4.5: Handler needs: source (skb->data), dest (user VA), len For copy_to_iter(src, bytes, iter), which arg is dest? FILL: regs->di = __, regs->si = __, regs->dx = __
COUNTER-QUESTION: _copy_to_iter copies TO iter, not FROM iter. iter describes user buffer. How to get user VA from iter? PARADOX: iter might be iovec, kvec, or ubuf. Which type for recv()?
dmesg shows: [COPY2] dev=lo skb_data=ffff8cb00df8acfe skb_len=66
Q5.1: COPY1 dest was 0xffff8cb08de63c00. COPY2 skb_data is 0xffff8cb00df8acfe. DIFFERENT! Q5.2: Why different? Kernel might allocate NEW skb for transmit? Q5.3: Or skb->data moves within same skb (skb_push/skb_pull)? Q5.4: Check loopback driver source: cat /home/r/Desktop/learn_kernel/source/drivers/net/loopback.c | head -100 Q5.5: Find loopback_xmit function. Does it clone skb or reuse? Q5.6: If skb is CLONED, is skb->data same or different?
COUNTER-QUESTION: skb_clone() shares skb->data. skb_copy() allocates new data. Which does loopback use? PARADOX: If loopback clones, COPY1 dest should equal COPY2 skb_data. But they differ.
skb_len=66 bytes
Q6.1: Your payload = 16 bytes (“HELLO_SEND_TRACE”) Q6.2: UDP header = 8 bytes → 16 + 8 = 24 Q6.3: IP header = 20 bytes → 24 + 20 = 44 Q6.4: Ethernet header = 14 bytes → 44 + 14 = 58 Q6.5: 66 - 58 = 8 bytes EXTRA. Where from? Q6.6: Ethernet minimum frame is 64 bytes. 58 < 64. Padding needed? Padding = 64 - 58 = 6 bytes? But you have 8 extra. Q6.7: Or is this FCS (CRC32 = 4 bytes)? 58 + 4 = 62. Still not 66. Q6.8: VLAN tag? 4 bytes. 58 + 4 = 62. Nope. Q6.9: Maybe skb_len includes something else. skb_headroom? skb_tailroom?
COUNTER-QUESTION: Does skb_len = just data, or data + headers? What about skb->len vs skb->data_len? PARADOX: You see 66, math says 58. 8 bytes unaccounted.
Q7.1: Sender buf PA = ??? (need to run sender with sudo + pagemap) Q7.2: Kernel skb PA = ??? (calculated from VA - page_offset_base) Q7.3: Receiver buf PA = ??? (need to run receiver with sudo + pagemap) Q7.4: All 3 should be DIFFERENT physical addresses. Verify. Q7.5: Can you dump physical RAM at those addresses? /dev/mem requires root. sudo xxd -s PA -l 16 /dev/mem But /dev/mem is restricted. Alternative? Q7.6: Use kernel module to read physical RAM: phys_to_virt(PA)->read
COUNTER-QUESTION: If you prove different PA for each, you prove 3 copies exist in RAM. But kernel might have freed sender’s buffer already. PARADOX: When sender exits, its pages are freed. Is sender buf still in RAM when receiver reads?
T01: Run sudo cat /proc/kallsyms | grep page_offset_base → record result
T02: Calculate: 0xffff8cb08de63c00 - page_offset_base = PA
T03: Create recv_trace_hw.c with kprobe on _copy_to_iter
T04: Run sender+receiver with BOTH kprobe modules loaded
T05: Capture COPY #4: source kernel VA, dest user VA, len
T06: Run sender with sudo, verify PA from pagemap
T07: Run receiver with sudo, verify PA from pagemap
T08: Explain why len=22,1,37 instead of 16
T09: Explain why COPY1 dest ≠ COPY2 skb_data
T10: Explain 66 bytes vs expected 58 bytes
F01: _copy_to_iter might be inlined → kprobe registration fails F02: iter type is ITER_UBUF but you read __iov → crash F03: Running pagemap without root → PFN = 0 F04: KASLR makes page_offset_base different from documentation F05: Receiver exits before you check dmesg → lose COPY #4 trace F06: Multiple packets on loopback → cannot identify YOUR packet
Grilling questions: 42 Counter-questions: 7 Paradoxes: 7 Sub-tasks: 10 Failure predictions: 6
TOTAL: 72 items to address before writing receive trace module.
NO SOLUTIONS WRITTEN. USER MUST CALCULATE AND FILL BLANKS.