[Valgrind-users] Can valgrind print the used "uninitialized value" and its size?

[ISHIKAWA,chiaki <ish...@yk...>]

Hi,

I was running mozilla TB and found a message during a test run


==20163== Conditional jump or move depends on uninitialised value(s)
==20163==    at 0x90974BC: nsJSObjWrapper::NP_SetProperty(NPObject*,
void*, _NPVariant const*) (nsJSNPRuntime.cpp:137)
  ...

so there is an uninitialized usage.

In this particular instance, I  realized that it would be really great
to know the value of this uninitialized memory (turns out to be
automatic variable on the stack.)
so that I can learn what has happened to the subsequent processing.
Can the uninitialized value was zero or not, for example?

Can valgrind print value of used uninitialized memory location, say,
something along the suggested manner below?

I use the above example:

  ==20163== Conditional jump or move depends on uninitialised value(s)
  ==20163== The value used was: 0xdeadbeaf (4 bytes)
  ==20163==    at 0x90974BC: nsJSObjWrapper::NP_SetProperty(NPObject*,
void*, _NPVariant const*) (nsJSNPRuntime.cpp:137)
  ...

or maybe considering the possibility of byte block:

  ==20163== Conditional jump or move depends on uninitialised value(s)
  ==20163== The value used started with 0xde 0xad 0xbe 0xaf 0xaa 0xbb
0xcc ... (up to the minimum of first N octets or the size of the data,
maybe N can be 16 , etc.)
  ==20163==    at 0x90974BC: nsJSObjWrapper::NP_SetProperty(NPObject*,
void*, _NPVariant const*) (nsJSNPRuntime.cpp:137)

I believe this simple addition makes the detection very useful
because it would make creating the suggested patch easier by knowing
why a program that has been used so long had a hidden issue for so many
years.

Thank you for sharing this great tool with the programming community.

Best Regards,
Chiaki Ishikawa