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 <bbol...@gmail.com
<mailto:bbol...@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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Dr. Benjamin Bolker
Professor, Mathematics & Statistics and Biology, McMaster University
Director, School of Computational Science and Engineering
(Acting) Graduate chair, Mathematics & Statistics
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