changes in R-devel and zero-extent objects in Rcpp
The ASAN errors occur *even if the zero-length object is not actually
accessed*/is used in a perfectly correct manner, i.e. it's perfectly
legal in base R to define `m <- numeric(0)` or `m <- matrix(nrow = 0,
ncol = 0)`, whereas doing the equivalent in Rcpp will (now) lead to an
ASAN error.
i.e., these are *not* previously cryptic out-of-bounds accesses that
are now being revealed, but instead sensible and previously legal
definitions of zero-length objects that are now causing problems.
I'm pretty sure I'm right about this, but it's absolutely possible
that I'm just confused at this point; I don't have a super-simple
example to show you at the moment. The closest is this example by Mikael
Jagan: https://github.com/lme4/lme4/issues/794#issuecomment-2155093049
which shows that if x is a pointer to a zero-length vector (in plain
C++ for R, no Rcpp is involved), DATAPTR(x) and REAL(x) evaluate to
different values.
Mikael further points out that "Rcpp seems to cast a (void *)
returned by DATAPTR to (double *) when constructing a Vector<REALSXP>
from a SEXP, rather than using the (double *) returned by REAL." So
perhaps R-core doesn't want to guarantee that these operations give
identical answers, in which case Rcpp will have to change the way it
does things ...
cheers
Ben
On 2024-06-08 6:39 p.m., Kevin Ushey wrote:
IMHO, this should be changed in both Rcpp and downstream packages:
1. Rcpp could check for out-of-bounds accesses in cases like these, and
emit an R warning / error when such an access is detected;
2. The downstream packages unintentionally making these out-of-bounds
accesses should be fixed to avoid doing that.
That is, I think this is ultimately a bug in the affected packages, but
Rcpp could do better in detecting and handling this for client packages
(avoiding a segfault).
Best,
Kevin
On Sat, Jun 8, 2024, 3:06?PM Ben Bolker <bbolker at gmail.com
<mailto:bbolker at gmail.com>> wrote:
? ? A change to R-devel (SVN r86629 or
https://github.com/r-devel/r-svn/commit/92c1d5de23c93576f55062e26d446feface07250 <https://github.com/r-devel/r-svn/commit/92c1d5de23c93576f55062e26d446feface07250>
has changed the handling of pointers to zero-length objects, leading to
ASAN issues with a number of Rcpp-based packages (the commit message
reads, in part, "Also define STRICT_TYPECHECK when compiling
inlined.c.")
? ?I'm interested in discussion from the community.
? ?Details/diagnosis for the issues in the lme4 package are here:
https://github.com/lme4/lme4/issues/794
<https://github.com/lme4/lme4/issues/794>, with a bit more discussion
about how zero-length objects should be handled.
? ?The short(ish) version is that r86629 enables the
CATCH_ZERO_LENGTH_ACCESS definition. This turns on the CHKZLN macro
<https://github.com/r-devel/r-svn/blob/4ef83b9dc3c6874e774195d329cbb6c11a71c414/src/main/memory.c#L4090-L4104 <https://github.com/r-devel/r-svn/blob/4ef83b9dc3c6874e774195d329cbb6c11a71c414/src/main/memory.c#L4090-L4104>>,
which returns a trivial pointer (rather than the data pointer that
would
be returned in the normal control flow) if an object has length 0:
/* Attempts to read or write elements of a zero length vector will
? ? result in a segfault, rather than read and write random memory.
? ? Returning NULL would be more natural, but Matrix seems to assume
? ? that even zero-length vectors have non-NULL data pointers, so
? ? return (void *) 1 instead. Zero-length CHARSXP objects still have a
? ? trailing zero byte so they are not handled. */
? ?In the Rcpp context this leads to an inconsistency, where `REAL(x)`
is a 'real' external pointer and `DATAPTR(x)` is 0x1, which in turn
leads to ASAN warnings like
runtime error: reference binding to misaligned address 0x000000000001
for type 'const double', which requires 8 byte alignment
0x000000000001: note: pointer points here
? ? I'm in over my head and hoping for insight into whether this
problem
should be resolved by changing R, Rcpp, or downstream Rcpp packages ...
? ?cheers
? ? Ben Bolker
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