CODEWITHSUNDEEP
segmentation-faultvalagenie
user2784930
user2784930

Reputation: 15

Genie Segfaults on easy typecast

I have Valac 0.30 installed. Running the following code, 

[indent=4]
init
    str : string
    str = "Hello World";
    data : array of uint8
    data = (array of uint8) str;
    print "%i\n", data.length;

I get a segfault. GDB tells me this: 

Program received signal SIGSEGV, Segmentation fault.
__memcpy_sse2_unaligned ()
    at ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S:36
36  ../sysdeps/x86_64/multiarch/memcpy-sse2-unaligned.S: No such file or directory.

I've seen some other people with this problem, but none of them got solutions which have worked for me.

Upvotes: 1

Views: 3891

Answers (1)

Jens Mühlenhoff
Jens Mühlenhoff

Reputation: 14873

You are telling the compiler to hard cast a string into an array of uint8, however those types are not assignment compatible.

Under the hood the simplified generated C code (which you can get with valac -C) looks like this:

#include <glib.h>

int main (void) {
        gchar* str = g_strdup ("Hello World");
        // Ouch: A negative number is used as length for g_memdup
        // This will produce a segfault, because the parameter is unsigned and will overflow to a very big number.
        // The string is only 11 bytes long however
        guint8* data = g_memdup (str, -1 * sizeof(guint8));
        int data_length1 = -1;
        g_print ("%i\n\n", data_length1);
        g_free (data);
        g_free (str);
        return 0;
}

The string data type has two properties that are meant for what you are trying to do (Vala syntax):

public int length { get; }
public uint8[] data { get; }

So you could write your code like this:

[indent=4]
init
    str: string = "Hello World";
    print "%i\n", str.length;

Or like this:

[indent=4]
init
    str: string = "Hello World";
    data: array of uint8 = str.data;
    print "%i\n", data.length;

For completeness here is the gdb backtrace:

(gdb) run
Starting program: /home/user/src/genie/Test
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Program received signal SIGSEGV, Segmentation fault.
__memcpy_avx_unaligned () at ../sysdeps/x86_64/multiarch/memcpy-avx-unaligned.S:245
245             vmovdqu -0x80(%rsi,%rdx), %xmm5
(gdb) bt
#0  __memcpy_avx_unaligned () at ../sysdeps/x86_64/multiarch/memcpy-avx-unaligned.S:245
#1  0x00007ffff78b66c6 in memcpy (__len=4294967295, __src=0x60cdd0, __dest=<optimized out>) at /usr/include/bits/string3.h:53
#2  g_memdup (mem=0x60cdd0, byte_size=4294967295) at /usr/src/debug/dev-libs/glib-2.46.2-r2/glib-2.46.2/glib/gstrfuncs.c:392
#3  0x00000000004007d6 in _vala_array_dup1 (self=0x60cdd0 "Hello World", length=-1) at /home/user/src/genie/Test.gs:6
#4  0x000000000040085e in _vala_main (args=0x7fffffffdf78, args_length1=1) at /home/user/src/genie/Test.gs:6
#5  0x00000000004008f5 in main (argc=1, argv=0x7fffffffdf78) at /home/user/src/genie/Test.gs:2

So g_memdup is trying to copy 4294967295 bytes from an 11 byte string here.

Upvotes: 2

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