Re: Better alternative to strncpy in QEMU.

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/04/21 06:51, Thomas Huth wrote:
>>
> 
> I think this is pretty much the same as g_strlcpy() from the glib:
> 
> https://developer.gnome.org/glib/2.66/glib-String-Utility-Functions.html#g-strlcpy 
> 
> So I guess Paolo had something different in mind when adding this task?

Yes, I did.  strncpy is used legitimately when placing data in a 
fixed-size buffer that is written to a socket, to a file or to guest 
memory.  The problem with using g_strlcpy in those cases is that it does 
not write past the first '\0' character, and therefore it can leak host 
data.

What I had in mind was basically strncpy plus an assertion that the last 
copied byte will be set to 0.  It can be written in many ways, for 
example strncpy followed by assert(dest[destlen - 1] == '\0'), or like 
assert(strlen(src) < destlen) followed by strncpy, or of course you 
could write a for loop by hand.

Once you do that, you can split uses of strncpy in two: those where the 
reader expects the last byte to be zero, and those where the reader does 
not.  (I don't expect many cases of the first type, because the reader 
always has to think of how to handle a malicious data stream that does 
not have a zero termination).

As long as you avoid the accidentally quadratic behavior that Peter 
pointed out, any way is fine since performance does not matter on these 
paths.  Making the code nice and readable is more important.

Paolo