On Thu, Aug 24, 2017 at 9:52 AM, Junio C Hamano <gitster@xxxxxxxxx> wrote:
> Brandon Casey <drafnel@xxxxxxxxx> writes:
>
>> Ah, you probably meant something like this:
>>
>> const char strbuf_slopbuf = '\0';
>>
>> which gcc will apparently place in the read-only segment. I did not know that.
>
> Yes but I highly suspect that it would be very compiler dependent
> and not something the language lawyers would recommend us to rely
> on.
I think compilers may have the option of placing variables that are
explicitly initialized to zero in the bss segment too, in addition to
those that are not explicitly initialized. So I agree that no one
should write code that depends on their variables being placed in one
segment or the other, but I could see someone using this behavior as
an additional safety check; kind of a free assert, aside from the
additional space in the .rodata segment.
> My response was primarily to answer "why?" with "because we did not
> bother". The above is a mere tangent, i.e. "multiple copies of
> empty strings is a horrible implementation (and there would be a way
> to do it with a single instance)".
Merely adding const to our current strbuf_slopbuf declaration does not
buy us anything since it will be allocated in r/w memory. i.e. it
would still be possible to modify it via the buf member of strbuf. So
you can't just do this:
const char strbuf_slopbuf[1];
That's pretty much equivalent to what we currently have since it only
restricts modifying the contents of strbuf_slopbuf directly through
the strbuf_slopbuf variable, but it does not restrict modifying it
through a pointer to that object.
Until yesterday, I was under the impression that the only way to
access data in the rodata segment was through a constant literal. So
my initial thought was that we could do something like:
const char * const strbuf_slopbuf = "";
..but that variable cannot be used in a static assignment like:
struct strbuf foo = {0, 0, (char*) strbuf_slopbuf};
So it seemed like our only option was to use a literal "" everywhere
instead of a slopbuf variable _if_ we wanted to have the guarantee
that our "slopbuf" could not be modified.
But what I learned yesterday, is that at least gcc/clang will place
the entire variable in the rodata segment if the variable is both
marked const _and_ initialized.
i.e. this will be allocated in the .rodata segment:
const char strbuf_slopbuf[1] = "";
>> #define STRBUF_INIT { .alloc = 0, .len = 0, .buf = (char*) &strbuf_slopbuf }
>>
>> respectively. Yeah, that's definitely preferable to a macro.
>> Something similar could be done in object.c.
>
> What is the main objective for doing this change? The "make sure we
> do not write into that slopbuf" assert() bothers you and you want to
> replace it with an address in the read-only segment?
Actually nothing about the patch bothers me. The point of that patch
is to make sure we don't accidentally modify the slopbuf. I was just
looking for a way for the compiler to help out and wondering if there
was a reason we didn't attempt to do so in the first place.
I think the main takeaway here is that I learned something yesterday
:-) I didn't actually intend to submit a patch for any of this, but
if anything useful came out of the discussion I thought Martin may
incorporate it into his patch if he wanted to.
-Brandon
