[R-pkg-devel] RFC: C backtraces for R CMD check via just-in-time debugging

[Ivan Krylov]

Vladimir,

Thank you for the example and for sharing the ideas regarding
symbol-relative offsets!

On Thu, 7 Mar 2024 09:38:18 -0500 (EST)
Vladimir Dergachev <volodya using mindspring.com> wrote:

>          unw_get_reg(&cursor, UNW_REG_IP, &pc);

Is it ever possible for unw_get_reg() to fail (return non-zero) for
UNW_REG_IP? The documentation isn't being obvious about this. Then
again, if the process is so damaged it cannot even read the instruction
pointer from its own stack frame, any attempts at self-debugging must
be doomed.

>    * this should work as a package, but I am not sure whether the
> offsets between package symbols and R symbols would be static or not.

Since package shared objects are mmap()ed into the address space and
(at least on Linux with ASLR enabled) mmap()s are supposed to be made
unpredictable, this offset ends up not being static. On Linux, R seems
to be normally built as a position-independent executable, so no matter
whether there is a libR.so, both the R base address and the package
shared object base address are randomised:

$ cat ex.c
#include <stdint.h>
#include <R.h>
void addr_diff(void) {
 ptrdiff_t diff = (char*)&addr_diff - (char*)&Rprintf;
 Rprintf("self - Rprintf = %td\n", diff);
}
$ R CMD SHLIB ex.c
$ R-dynamic -q -s -e 'dyn.load("ex.so"); .C("addr_diff");'
self - Rprintf = -9900928
$ R-dynamic -q -s -e 'dyn.load("ex.so"); .C("addr_diff");'
self - Rprintf = -15561600
$ R-static -q -s -e 'dyn.load("ex.so"); .C("addr_diff");'
self - Rprintf = 45537907472976
$ R-static -q -s -e 'dyn.load("ex.so"); .C("addr_diff");'
self - Rprintf = 46527711447632

>    * R ought to know where packages are loaded, we might want to be
> clever and print out information on which package contains which
> function, or there might be identical R_init_RMVL() printouts.

That's true. Informaion on all registered symbols is available from
getLoadedDLLs().

-- 
Best regards,
Ivan