Assertion failed in transfer_fifo()

Host environment

• Operating system: Ubuntu 20.04
• OS/kernel version: Linux 5.15.0-56-generic
• Architecture: x86
• QEMU flavor: qemu-system-aarch64
• QEMU version: 7.2.50
• QEMU command line:

  export QEMU=/path/to/qemu-system-aarch64
  
  cat << EOF | $QEMU \
  -machine xlnx-zcu102 -monitor none -serial none \
  -display none -nodefaults -qtest stdio -audio none
  writel 0xff070000 0x0f73720a
  writel 0xff07003c 0x1f37ee63
  EOF

Description of problem

In transfer_fifo(), fifo32_pop() fails since less than 32 bytes are in the fifo.

Steps to reproduce

export QEMU=/path/to/qemu-system-aarch64

cat << EOF | $QEMU \
-machine xlnx-zcu102 -monitor none -serial none \
-display none -nodefaults -qtest stdio -audio none
writel 0xff070000 0x0f73720a
writel 0xff07003c 0x1f37ee63
EOF

Additional information

==31717==WARNING: ASan doesn't fully support makecontext/swapcontext functions and may produce false positives in some cases!
INFO: found LLVMFuzzerCustomMutator (0x55871da359f0). Disabling -len_control by default.
INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 1734665286
INFO: Loaded 1 modules   (618606 inline 8-bit counters): 618606 [0x558720b94000, 0x558720c2b06e), 
INFO: Loaded 1 PC tables (618606 PCs): 618606 [0x558720222e60,0x558720b93540), 
/root/videzzo/videzzo_qemu/out-san/qemu-videzzo-aarch64-target-videzzo-fuzz-xlnx-zynqmp-can: Running 1 inputs 1 time(s) each.
INFO: Reading pre_seed_input if any ...
INFO: Executing pre_seed_input if any ...
Matching objects by name , *xlnx.zynqmp-can*
This process will fuzz the following MemoryRegions:
  * xlnx.zynqmp-can[1] (size 84)
  * xlnx.zynqmp-can[0] (size 84)
  * xlnx.zynqmp-can[1] (size 84)
  * xlnx.zynqmp-can[0] (size 84)
This process will fuzz through the following interfaces:
  * clock_step, EVENT_TYPE_CLOCK_STEP, 0xffffffff +0xffffffff, 255,255
  * xlnx.zynqmp-can, EVENT_TYPE_MMIO_READ, 0xff070000 +0x84, 4,4
  * xlnx.zynqmp-can, EVENT_TYPE_MMIO_WRITE, 0xff070000 +0x84, 4,4
  * xlnx.zynqmp-can, EVENT_TYPE_MMIO_READ, 0xff060000 +0x84, 4,4
  * xlnx.zynqmp-can, EVENT_TYPE_MMIO_WRITE, 0xff060000 +0x84, 4,4
INFO: A corpus is not provided, starting from an empty corpus
#2      INITED cov: 3 ft: 4 corp: 1/1b exec/s: 0 rss: 491Mb
Running: poc-qemu-videzzo-aarch64-target-videzzo-fuzz-xlnx-zynqmp-can-crash-97ef02583c679111ba6ad823f573f139fac7c72e
qemu-videzzo-aarch64-target-videzzo-fuzz-xlnx-zynqmp-can: ../util/fifo8.c:62: uint8_t fifo8_pop(Fifo8 *): Assertion `fifo->num > 0' failed.
==31717== ERROR: libFuzzer: deadly signal
    #0 0x558718e0e10e in __sanitizer_print_stack_trace /root/llvm-project/compiler-rt/lib/asan/asan_stack.cpp:86:3
    #1 0x558718d5cd81 in fuzzer::PrintStackTrace() /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerUtil.cpp:210:38
    #2 0x558718d35cb6 in fuzzer::Fuzzer::CrashCallback() (.part.0) /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:236:18
    #3 0x558718d35d82 in fuzzer::Fuzzer::CrashCallback() /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:208:1
    #4 0x558718d35d82 in fuzzer::Fuzzer::StaticCrashSignalCallback() /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:207:19
    #5 0x7f3ad4eba41f  (/lib/x86_64-linux-gnu/libpthread.so.0+0x1441f)
    #6 0x7f3ad4ccc00a in __libc_signal_restore_set /build/glibc-SzIz7B/glibc-2.31/signal/../sysdeps/unix/sysv/linux/internal-signals.h:86:3
    #7 0x7f3ad4ccc00a in raise /build/glibc-SzIz7B/glibc-2.31/signal/../sysdeps/unix/sysv/linux/raise.c:48:3
    #8 0x7f3ad4cab858 in abort /build/glibc-SzIz7B/glibc-2.31/stdlib/abort.c:79:7
    #9 0x7f3ad4cab728 in __assert_fail_base /build/glibc-SzIz7B/glibc-2.31/assert/assert.c:92:3
    #10 0x7f3ad4cbcfd5 in __assert_fail /build/glibc-SzIz7B/glibc-2.31/assert/assert.c:101:3
    #11 0x55871d6eeac9 in fifo8_pop /root/videzzo/videzzo_qemu/qemu/build-san-6/../util/fifo8.c:62:5
    #12 0x55871a33f303 in fifo32_pop /root/videzzo/videzzo_qemu/qemu/include/qemu/fifo32.h:137:17
    #13 0x55871a334bb5 in transfer_fifo /root/videzzo/videzzo_qemu/qemu/build-san-6/../hw/net/can/xlnx-zynqmp-can.c:455:23
    #14 0x55871a32d4c0 in can_tx_post_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../hw/net/can/xlnx-zynqmp-can.c:830:9
    #15 0x558719393dcb in register_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../hw/core/register.c:122:9
    #16 0x558719397de8 in register_write_memory /root/videzzo/videzzo_qemu/qemu/build-san-6/../hw/core/register.c:203:5
    #17 0x55871c9e9073 in memory_region_write_accessor /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/memory.c:492:5
    #18 0x55871c9e89b1 in access_with_adjusted_size /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/memory.c:554:18
    #19 0x55871c9e72d6 in memory_region_dispatch_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/memory.c:1514:16
    #20 0x55871ca7548e in flatview_write_continue /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/physmem.c:2825:23
    #21 0x55871ca635cb in flatview_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/physmem.c:2867:12
    #22 0x55871ca63088 in address_space_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../softmmu/physmem.c:2963:18
    #23 0x558718e4e0cb in qemu_writel /root/videzzo/videzzo_qemu/qemu/build-san-6/../tests/qtest/videzzo/videzzo_qemu.c:1081:5
    #24 0x558718e4c544 in dispatch_mmio_write /root/videzzo/videzzo_qemu/qemu/build-san-6/../tests/qtest/videzzo/videzzo_qemu.c:1222:28
    #25 0x55871da313af in videzzo_dispatch_event /root/videzzo/videzzo.c:1122:5
    #26 0x55871da2872b in __videzzo_execute_one_input /root/videzzo/videzzo.c:272:9
    #27 0x55871da28600 in videzzo_execute_one_input /root/videzzo/videzzo.c:313:9
    #28 0x558718e5510c in videzzo_qemu /root/videzzo/videzzo_qemu/qemu/build-san-6/../tests/qtest/videzzo/videzzo_qemu.c:1497:12
    #29 0x55871da35c92 in LLVMFuzzerTestOneInput /root/videzzo/videzzo.c:1891:18
    #30 0x558718d36826 in fuzzer::Fuzzer::ExecuteCallback(unsigned char*, unsigned long) /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:594:17
    #31 0x558718d19454 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:323:21
    #32 0x558718d243fe in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char*, unsigned long)) /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:885:19
    #33 0x558718d109e6 in main /root/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:30
    #34 0x7f3ad4cad082 in __libc_start_main /build/glibc-SzIz7B/glibc-2.31/csu/../csu/libc-start.c:308:16
    #35 0x558718d10a3d in _start (/root/videzzo/videzzo_qemu/out-san/qemu-videzzo-aarch64-target-videzzo-fuzz-xlnx-zynqmp-can+0x3291a3d)

NOTE: libFuzzer has rudimentary signal handlers.
      Combine libFuzzer with AddressSanitizer or similar for better crash reports.
SUMMARY: libFuzzer: deadly signal
MS: 0 ; base unit: 0000000000000000000000000000000000000000