[Valgrind-users] Query about Invalid reads

[Andrew C. <and...@ci...>]

Hello,

I am using valgrind-3.6.0.SVN-Debian, and cant work out why I am getting
Invalid read errors.

The routine is one which is adding symbols to a std::map, indexed by
their virtual address.  As multiple symbols may share the same virtual
address, I need to concatenate the symbol names in the already present
symbol entry.

I have the following (cut down, debugging) code which demonstrates the
problem:

            Symbol * prev = prev_itt->second;
            char * prev_name = prev->name;

            size_t newsize = strlen(prev_name);
            newsize += strlen(sym->name);
            newsize += 2;


            prev->name = new char[newsize];
            //Should concat and delete prev_name, but for debugging
leave like this
            delete [] prev->name;

            //For debugging, put this back, so no leaks
            prev->name = prev_name;

With the new char[newsize] and delete [] prev->name commented out,
valgrind is perfectly happy with the code (which is effectively a nop)

However, with the new and delete present as above, valigrind complains with:

==10046== Invalid read of size 1
==10046==    at 0x4C286E4: __GI_strlen (mc_replace_strmem.c:284)
==10046==    by 0x40CB88: SymbolTable::insert(Symbol*) (symbol-table.cpp:37)
==10046==    by 0x406536: parse_symbol_file(std::string) (main.cpp:44)
==10046==    by 0x406DA4: main (main.cpp:70)
==10046==  Address 0x7ae7d36 is 0 bytes after a block of size 6 alloc'd
==10046==    at 0x4C27939: operator new[](unsigned long)
(vg_replace_malloc.c:305)
==10046==    by 0x40C969: Symbol::Symbol(unsigned long long, char, char
const*) (symbol.cpp:9)
==10046==    by 0x406526: parse_symbol_file(std::string) (main.cpp:44)
==10046==    by 0x406DA4: main (main.cpp:70)
==10046==
==10046== Invalid read of size 1
==10046==    at 0x4C286E4: __GI_strlen (mc_replace_strmem.c:284)
==10046==    by 0x40CB94: SymbolTable::insert(Symbol*) (symbol-table.cpp:38)
==10046==    by 0x406536: parse_symbol_file(std::string) (main.cpp:44)
==10046==    by 0x406DA4: main (main.cpp:70)
==10046==  Address 0x7ae7fe6 is 0 bytes after a block of size 6 alloc'd
==10046==    at 0x4C27939: operator new[](unsigned long)
(vg_replace_malloc.c:305)
==10046==    by 0x40C969: Symbol::Symbol(unsigned long long, char, char
const*) (symbol.cpp:9)
==10046==    by 0x406526: parse_symbol_file(std::string) (main.cpp:44)
==10046==    by 0x406DA4: main (main.cpp:70)

Where the two errors are referring to the two strlen() calls when
calculating newsize.

Are these errors indicating a supposed bug in my code, or are they
complaining about something in the __GI_strlen replaced code.  If so,
does this mean there is a bug in __GI_strlen ?

Thanks in advance,

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com