Re: [PATCH] pch: Fix streaming of strings with embedded null bytes

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Oct 19, 2022 at 01:17:02PM +0100, Richard Sandiford wrote:
> Jakub Jelinek <jakub@redhat.com> writes:
> > On Wed, Oct 19, 2022 at 12:54:11PM +0100, Richard Sandiford via Gcc-patches wrote:
> >> Lewis Hyatt via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> >> > When a GTY'ed struct is streamed to PCH, any plain char* pointers it contains
> >> > (whether they live in GC-controlled memory or not) will be marked for PCH
> >> > output by the routine gt_pch_note_object in ggc-common.cc. This routine
> >> > special-cases plain char* strings, and in particular it uses strlen() to get
> >> > their length. Thus it does not handle strings with embedded null bytes, but it
> >> > is possible for something PCH cares about (such as a string literal token in a
> >> > macro definition) to contain such embedded nulls. To fix that up, add a new
> >> > GTY option "string_length" so that gt_pch_note_object can be informed the
> >> > actual length it ought to use, and use it in the relevant libcpp structs
> >> > (cpp_string and ht_identifier) accordingly.
> >> 
> >> This isn't really my area, as I'm about to demonstrate with this
> >> question, but: regarding
> >> 
> >>   if (note_ptr_fn == gt_pch_p_S)
> >>     (*slot)->size = strlen ((const char *)obj) + 1;
> >>   else
> >>     (*slot)->size = ggc_get_size (obj);
> >> 
> >> do you know why the PCH code goes out of its way to handle the sizes of
> >> strings specially?  Are there enough garbage strings in the string pool
> >> that it's worth optimising the size of the saved memory for strings but
> >> not for other types of object?  Or is the gt_pch_p_S test needed for
> >> correctness, rather than just being an optimisation?
> >
> > Just guessing, not all GC strings live in the stringpool.
> > Isn't e.g. ggc_strdup just a GC allocation where the string length
> > isn't stored anywhere?
> 
> Is that different from other GC VLA allocations though?  I thought
> ultimately we just tried to save and restore the containing pages.

I think just the objects in it, not entire pages (ggc_get_size (obj)
sized chunks for non-strings).

> > And sometimes it isn't even GC allocated, e.g. ggc_strdup ("") just
> > returns ""; I guess const char * pointers in GC memory can also point
> > to string literals in .rodata and for PCH we move them.
> 
> Ah, OK, that would definitely explain it, thanks.  In that case,
> are you OK with the patch, as a way of continuing to support rodata
> string pointers while also allowing embedded nuls?

LGTM.

	Jakub