Stack smash and sscanf

Question

reply is S|[2 3 4 5 6 7 8 9]|[2 3 4 5 6 7 8 9]

char com[10], f[100], s[100];
sscanf(reply, "%[^!]|%[^!]|%[^!]", com, f, s);

It causes stack smash. I know that sscanf is usually unsafe, but I'm wondering why it fail here - when input string in fine.

Here's output:

* stack smashing detected *: ./testClient terminated ======= Backtrace: ========= /lib/i386-linux-gnu/libc.so.6(__fortify_fail+0x50)[0x1f5df0] /lib/i386-linux-gnu/libc.so.6(+0xe5d9a)[0x1f5d9a] ./testClient[0x804b336] /lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x126e37] ./testClient[0x804add1]

======= Memory map: ======== 00110000-0026a000 r-xp 00000000 08:01 523281 /lib/i386-linux-gnu/libc-2.13.so 0026a000-0026b000 ---p 0015a000 08:01 523281 /lib/i386-linux-gnu/libc-2.13.so 0026b000-0026d000 r--p 0015a000 08:01 523281
/lib/i386-linux-gnu/libc-2.13.so 0026d000-0026e000 rw-p 0015c000 08:01 523281 /lib/i386-linux-gnu/libc-2.13.so 0026e000-00271000 rw-p 00000000 00:00 0 00271000-00286000 r-xp 00000000 08:01 523299
/lib/i386-linux-gnu/libpthread-2.13.so 00286000-00287000 r--p 00015000 08:01 523299 /lib/i386-linux-gnu/libpthread-2.13.so 00287000-00288000 rw-p 00016000 08:01 523299
/lib/i386-linux-gnu/libpthread-2.13.so 00288000-0028a000 rw-p 00000000 00:00 0 003e8000-00404000 r-xp 00000000 08:01 523304
/lib/i386-linux-gnu/ld-2.13.so 00404000-00405000 r--p 0001b000 08:01 523304 /lib/i386-linux-gnu/ld-2.13.so 00405000-00406000 rw-p 0001c000 08:01 523304 /lib/i386-linux-gnu/ld-2.13.so 004c9000-004d0000 r-xp 00000000 08:01 523283
/lib/i386-linux-gnu/librt-2.13.so 004d0000-004d1000 r--p 00006000 08:01 523283 /lib/i386-linux-gnu/librt-2.13.so 004d1000-004d2000 rw-p 00007000 08:01 523283 /lib/i386-linux-gnu/librt-2.13.so 0053f000-00540000 r-xp 00000000 00:00 0 [vdso] 007c9000-007ed000 r-xp 00000000 08:01 523303
/lib/i386-linux-gnu/libm-2.13.so 007ed000-007ee000 r--p 00023000 08:01 523303 /lib/i386-linux-gnu/libm-2.13.so 007ee000-007ef000 rw-p 00024000 08:01 523303 /lib/i386-linux-gnu/libm-2.13.so 00cee000-00dcd000 r-xp 00000000 08:01 1051412
/usr/lib/i386-linux-gnu/libstdc++.so.6.0.14 00dcd000-00dd1000 r--p 000de000 08:01 1051412 /usr/lib/i386-linux-gnu/libstdc++.so.6.0.14 00dd1000-00dd2000 rw-p 000e2000 08:01 1051412
/usr/lib/i386-linux-gnu/libstdc++.so.6.0.14 00dd2000-00dd9000 rw-p 00000000 00:00 0 00e51000-00e53000 r-xp 00000000 08:01 1070941
/usr/lib/libboost_system.so.1.46.1 00e53000-00e54000 r--p 00002000 08:01 1070941 /usr/lib/libboost_system.so.1.46.1 00e54000-00e55000 rw-p 00003000 08:01 1070941 /usr/lib/libboost_system.so.1.46.1 00e9c000-00eb6000 r-xp 00000000 08:01 523308
/lib/i386-linux-gnu/libgcc_s.so.1 00eb6000-00eb7000 r--p 00019000 08:01 523308 /lib/i386-linux-gnu/libgcc_s.so.1 00eb7000-00eb8000 rw-p 0001a000 08:01 523308 /lib/i386-linux-gnu/libgcc_s.so.1 08048000-08067000 r-xp 00000000 08:01 1591111
/home/alex/pj/cpp/testClient/bin/Debug/testClient 08067000-08068000 r--p 0001e000 08:01 1591111
/home/alex/pj/cpp/testClient/bin/Debug/testClient 08068000-08069000 rw-p 0001f000 08:01 1591111
/home/alex/pj/cpp/testClient/bin/Debug/testClient 09050000-09071000 rw-p 00000000 00:00 0 [heap] b78d9000-b78dd000 rw-p 00000000 00:00 0 b78ef000-b78f2000 rw-p 00000000 00:00 0 bfba9000-bfbca000 rw-p 00000000 00:00 0 [stack]

Jonathan Leffler Jonathan Leffler · Accepted Answer · 2011-09-15T19:39:12

One problem is that you have pipe | characters separating the strings in your data but your sscanf() format looks for the character class [^!] (all except exclamation marks). You should also specify the size of the buffer for the character classes, and check the return value:

char com[10], f[100], s[100];
if (sscanf(reply, "%9[^|]|%99[^|]|%99[^|]", com, f, s) != 3)
    ...format error...

Note the use of 9 and 99; you must specify the size leaving space for a terminal null.

Stack smash and sscanf

1 Answers