Re: [Valgrind-users] Invalid read well inside a block alloc'd

[Tom H. <to...@co...>]

On 14/01/2020 16:53, Paul-Antoine Arras wrote:

> I'm stumbling upon a weird message from Valgrind when run on my 
> application as follows:
> 
> $ valgrind --vgdb=yes --vgdb-error=0 --undef-value-errors=no $my_app
> 
> So Valgrind reports:
> 
> ==1644== Thread 9:
> ==1644== Invalid read of size 8
> ==1644==    at 0x4A39B40: PR_int__give_lang_env_for_slave (PR__int.c:348)
> ==1644==  Address 0x12d152c8 is 24 bytes inside a block of size 104 alloc'd
> ==1644==    at 0x483577F: malloc (vg_replace_malloc.c:309)
> ==1644==    by 0x4A3C4B4: [...]
> 
> I struggle to understand how a read into a block of properly alloc'd 
> memory can be invalid, given that the application doesn't use client 
> requests.
> To be sure, I double-checked the status of the entire buffer under vgdb:
> 
> (gdb) mo xb 0x12d152b0 104
> [...]
> Address 0x12D152B0 len 104 has 104 bytes unaddressable
> 
> How can a block of dynamically-allocated memory be marked unaddressable 
> without having been freed?

By using the VALGRIND_MAKE_MEM_NOACCESS macro.

Tom

-- 
Tom Hughes (to...@co...)
http://compton.nu/