[cfe-dev] [llvm-dev] clang and -D_FORTIFY_SOURCE=1

[Martin Storsjö via cfe-dev]

On Wed, 4 Dec 2019, Serge Guelton via llvm-dev wrote:

> > Are you sure you've diagnosed the issue correctly? __builtin___memcpy_chk
> works correctly, as far as I know.
> 100% sure. Let's have a look at the output of
> 
>   #include <string.h>
>   static char dest[10];
>   char* square(int n) {
>     memcpy(dest, "hello", n);
>     return dest;
>   }
> 
> compiled with -D_FORTIFY_SOURCE=1 -O1 : https://godbolt.org/z/UvABWp
> 
> Clang issues a call to memcpy, while gcc issues a call to __memcpy_chk.
> The call to __memcpy_chk performs extra runtime checks memcpy doesn't,
> and clang doesn't generate the extra checks inline either. This is a
> separate
> concern from the accuracy of __builtin_object_size, just a different runtime
> behavior.
> 
> Clang could generate the call to __memcpy_chk if its declaration is
> available, which is the case for the glibc.

No, this doesn't seem to be the case.

I expanded your testcase by removing the string.h include and replacing it 
with the relevant bits of it:

https://godbolt.org/z/NKUQbW

The regular call to memcpy ends up without the extra safety checks and 
just generates a call to memcpy. But if I call __builtin___memcpy_chk 
directly, it does generate a call to __memcpy_chk.

So I would conclude that the builtin works just fine, but the issue is 
with the exact rules for when and how the inline function is preferred 
over the externally available version.

I added a "#define VISIBIILTY extern" to ease testing and changing it. If 
you change that into static, you'll see that the inline version of the 
function actually ends up used, and it does generate a call to 
__memcpy_chk even there.

And this does seem to show that __builtin_object_size does work fine from 
within an inlined function now, contrary to my earlier comments.

// Martin