$ ./img2sixel -o ./a.sixel -8 -p 1 -C 10 -d fs -f auto -s auto -t auto -E ~/Downloads/poc
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==3495149==ERROR: AddressSanitizer: heap-use-after-free on address 0x6080000000a0 at pc 0x7ffff74e92cd bp 0x7fffffff84d0 sp 0x7fffffff84c0
READ of size 4 at 0x6080000000a0 thread T0
#0 0x7ffff74e92cc in sixel_dither_unref /home/wulearn/Desktop/libsixel/src/dither.c:388
#1 0x7ffff7537374 in sixel_encoder_encode_frame /home/wulearn/Desktop/libsixel/src/encoder.c:1079
#2 0x7ffff753b0af in load_image_callback /home/wulearn/Desktop/libsixel/src/encoder.c:1679
#3 0x7ffff7531302 in load_gif /home/wulearn/Desktop/libsixel/src/fromgif.c:671
#4 0x7ffff752abc9 in load_with_builtin /home/wulearn/Desktop/libsixel/src/loader.c:908
#5 0x7ffff752b5cb in sixel_helper_load_image_file /home/wulearn/Desktop/libsixel/src/loader.c:1418
#6 0x7ffff753b513 in sixel_encoder_encode /home/wulearn/Desktop/libsixel/src/encoder.c:1743
#7 0x555555558a3b in main /home/wulearn/Desktop/libsixel/converters/img2sixel.c:457
#8 0x7ffff72c60b2 in __libc_start_main (/usr/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
#9 0x55555555638d in _start (/home/wulearn/Desktop/libsixel/converters/.libs/img2sixel+0x238d)
0x6080000000a0 is located 0 bytes inside of 94-byte region [0x6080000000a0,0x6080000000fe)
freed by thread T0 here:
#0 0x7ffff76a27cf in __interceptor_free (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x10d7cf)
#1 0x7ffff7549523 in sixel_allocator_free /home/wulearn/Desktop/libsixel/src/allocator.c:230
#2 0x7ffff74e9226 in sixel_dither_destroy /home/wulearn/Desktop/libsixel/src/dither.c:368
#3 0x7ffff74e92f1 in sixel_dither_unref /home/wulearn/Desktop/libsixel/src/dither.c:389
#4 0x7ffff75352bf in sixel_encoder_prepare_palette /home/wulearn/Desktop/libsixel/src/encoder.c:584
#5 0x7ffff75369a4 in sixel_encoder_encode_frame /home/wulearn/Desktop/libsixel/src/encoder.c:981
#6 0x7ffff753b0af in load_image_callback /home/wulearn/Desktop/libsixel/src/encoder.c:1679
#7 0x7ffff7531302 in load_gif /home/wulearn/Desktop/libsixel/src/fromgif.c:671
#8 0x7ffff752abc9 in load_with_builtin /home/wulearn/Desktop/libsixel/src/loader.c:908
#9 0x7ffff752b5cb in sixel_helper_load_image_file /home/wulearn/Desktop/libsixel/src/loader.c:1418
#10 0x7ffff753b513 in sixel_encoder_encode /home/wulearn/Desktop/libsixel/src/encoder.c:1743
#11 0x555555558a3b in main /home/wulearn/Desktop/libsixel/converters/img2sixel.c:457
#12 0x7ffff72c60b2 in __libc_start_main (/usr/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
previously allocated by thread T0 here:
#0 0x7ffff76a2bc8 in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x10dbc8)
#1 0x555555558c4e in rpl_malloc /home/wulearn/Desktop/libsixel/converters/malloc_stub.c:45
#2 0x7ffff7549243 in sixel_allocator_malloc /home/wulearn/Desktop/libsixel/src/allocator.c:162
#3 0x7ffff74e8a7a in sixel_dither_new /home/wulearn/Desktop/libsixel/src/dither.c:306
#4 0x7ffff7535133 in sixel_encoder_prepare_palette /home/wulearn/Desktop/libsixel/src/encoder.c:570
#5 0x7ffff75369a4 in sixel_encoder_encode_frame /home/wulearn/Desktop/libsixel/src/encoder.c:981
#6 0x7ffff753b0af in load_image_callback /home/wulearn/Desktop/libsixel/src/encoder.c:1679
#7 0x7ffff7531302 in load_gif /home/wulearn/Desktop/libsixel/src/fromgif.c:671
#8 0x7ffff752abc9 in load_with_builtin /home/wulearn/Desktop/libsixel/src/loader.c:908
#9 0x7ffff752b5cb in sixel_helper_load_image_file /home/wulearn/Desktop/libsixel/src/loader.c:1418
#10 0x7ffff753b513 in sixel_encoder_encode /home/wulearn/Desktop/libsixel/src/encoder.c:1743
#11 0x555555558a3b in main /home/wulearn/Desktop/libsixel/converters/img2sixel.c:457
#12 0x7ffff72c60b2 in __libc_start_main (/usr/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
SUMMARY: AddressSanitizer: heap-use-after-free /home/wulearn/Desktop/libsixel/src/dither.c:388 in sixel_dither_unref
Shadow bytes around the buggy address:
0x0c107fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c107fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c107fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c107fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c107fff8000: fa fa fa fa 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c107fff8010: fa fa fa fa[fd]fd fd fd fd fd fd fd fd fd fd fd
0x0c107fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c107fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c107fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c107fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c107fff8060: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
Shadow gap: cc
==3495149==ABORTING
Hi,I found a heap-use-after-free in the current master 6a5be8b I build img2sixel with ASAN,this is ASAN report.
OS: Ubuntu 20.04.3 LTS x86_64 Kernel: 5.11.0-27-generic
POC: poc.zip