changes in R-devel and zero-extent objects in Rcpp
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> wrote:
A change to R-devel (SVN r86629 or
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, 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>,
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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