[Rd] Should last default to .Machine$integer.max-1 for substring()

brodie gaslam
Mon, 21 Jun 2021 01:28:12 -0000

> On Sunday, June 20, 2021, 6:21:22 PM EDT, Michael Chirico
<michaelchirico4 at gmail.com> wrote:
>
> Currently, substring defaults to last=1000000L, which strongly
> suggests the intent is to default to "nchar(x)" without having to
> compute/allocate that up front.
>
> Unfortunately, this default makes no sense for "very large"
strings
> which may exceed 1000000L in "width".
>
> The max width of a string is .Machine$integer.max-1:

I think the max width is .Machine$integer.max.? What happened below is a
bug due to buffer overflow in `strrep`:

> # works
> x = strrep(" ", .Machine$integer.max-1L)
> # fails
> x = strrep(" ", .Machine$integer.max)
> Error in strrep(" ", .Machine$integer.max) :
>?? 'Calloc' could not allocate memory (18446744071562067968 of 1
bytes)

Notice the very large number that was tried to be Calloc'ed.? That's
(size_t) -1.

The problem is (src/include/R_ext/RS.h at 85):

??? #define CallocCharBuf(n) (char *) R_chk_calloc((R_SIZE_T) ((n)+1),
sizeof(char))

The `((n) + 1)` overflows `int` and produces -1 (well, undefined behavior
so who knows), which when cast to size_t produces that very large number
which can't be allocated.

I think this should be:

??? #define CallocCharBuf(n) (char *) R_chk_calloc(((R_SIZE_T)(n))+1,
sizeof(char))

I can reproduce the failure before the change.? After the change I get:

??? > x = strrep(" ", .Machine$integer.max)
??? Error in strrep(" ", .Machine$integer.max) :
????? 'Calloc' could not allocate memory (2147483648 of 1 bytes)

I believe this to be the expected result on a machine that doesn't have
enough memory to allocate INT_MAX + 1 bytes, as happens to be the case on
my R build system (it's a VM that gets 2GB total as the host machine can
barely spare that to begin with).

> (see also the comment in src/main/character.c: "Character strings in R
> are less than 2^31-1 bytes, so we use int not size_t.")

FWIW WRE states:

> Note that R character strings are restricted to 2^31 - 1 bytes

This is INT_MAX or .Machine$integer.max, at least on machines for which
`int` is 32 bits, which I think typical for machines R builds on.?? From
having looked at the code a while ago I think WRE is right (so maybe the
comment in the code is wrong), but it was a while ago and I haven't tried
to allocate an INT_MAX long string.

Sorry this doesn't answer your original question.

Best,

Brodie.

>
>
> So it seems to me either .Machine$integer.max or
> .Machine$integer.max-1L would be a more sensible default. Am I missing
> something?
>
> Mike C
>
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