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

[Richard Sandiford <richard.sandiford@arm.com>]

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.

> 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?

Richard