[R-pkg-devel] Issue with flang-new (segfault from C stack overflow)

[Tomas Kalibera]

On 12/18/23 15:09, Ivan Krylov wrote:
> В Mon, 18 Dec 2023 11:06:16 +0100
> Jisca Huisman <jisca.huisman using gmail.com> пишет:
>
>> I isolated the problem in a minimal working example available here:
>> https://github.com/JiscaH/flang_segfault_min_example . All that does
>> is pass a vector of length N*N back and forth between R and Fortran.
>> It works fine for very long vectors (tested up to length 5e8), but
>> throws a segfault when I reshape a large array in Fortran to a vector
>> to pass to R, both when using RESHAPE() and when using loops.
> You've done an impressive amount of investigative work. Thank you for
> reducing your problem to such a small example! My eyes are drawn to
> these two lines:
>
>>>   integer, intent(IN) :: N
>>>   integer :: M(N,N)
> If this was C, such a declaration would mean a variable-length array
> that would have to be placed on the (limited-size) stack and eventually
> overflow it. gfortran places the array on the heap, so the program
> works:
>
>    integer, intent(IN) :: N
>    integer, intent(INOUT) :: V(N*N)
>    integer :: M(N,N)
>      1205:       48 63 db                movslq %ebx,%rbx
>      1208:       b8 00 00 00 00          mov    $0x0,%eax
>      120d:       48 85 db                test   %rbx,%rbx
>      1210:       49 89 c4                mov    %rax,%r12
>      1213:       4c 0f 49 e3             cmovns %rbx,%r12
>      1217:       48 89 df                mov    %rbx,%rdi
>      121a:       49 0f af fc             imul   %r12,%rdi
>      121e:       48 85 ff                test   %rdi,%rdi
>      1221:       48 0f 48 f8             cmovs  %rax,%rdi
>      1225:       48 c1 e7 02             shl    $0x2,%rdi
>      1229:       b8 01 00 00 00          mov    $0x1,%eax
>      122e:       48 0f 44 f8             cmove  %rax,%rdi
>      1232:       e8 19 fe ff ff          callq  1050 <malloc using plt>
>      1237:       48 89 c5                mov    %rax,%rbp
>      123a:       4c 89 e7                mov    %r12,%rdi
>      123d:       48 f7 d7                not    %rdi
>
> (Looking at the address of M in GDB and comparing it with the output
> of info proc mappings, I can confirm that it lives on the heap.)
>
> flang-new makes M into a C-style VLA:
>
>    integer, intent(IN) :: N
>    integer, intent(INOUT) :: V(N*N)
>    integer :: M(N,N)
>      74ec:       48 63 17                movslq (%rdi),%rdx
>      74ef:       89 d1                   mov    %edx,%ecx
>      74f1:       31 c0                   xor    %eax,%eax
>      74f3:       48 85 d2                test   %rdx,%rdx
>      74f6:       48 0f 49 c2             cmovns %rdx,%rax
>      74fa:       48 89 85 b0 fe ff ff    mov    %rax,-0x150(%rbp)
>      7501:       48 89 c2                mov    %rax,%rdx
>      7504:       48 0f af d2             imul   %rdx,%rdx
>      7508:       48 8d 34 95 0f 00 00    lea    0xf(,%rdx,4),%rsi
>      750f:       00
>      7510:       48 83 e6 f0             and    $0xfffffffffffffff0,%rsi
>      7514:       48 89 e2                mov    %rsp,%rdx
>      7517:       48 29 f2                sub    %rsi,%rdx
>      751a:       48 89 95 b8 fe ff ff    mov    %rdx,-0x148(%rbp)
>      7521:       48 89 d4                mov    %rdx,%rsp
>
> (Looking at the value of the stack pointer in GDB after M(N,N) is
> declared, I can see it way below the end of the stack and the loaded
> shared libraries according to info proc mappings. GDB doesn't let me
> see the address of M. The program crashes in `M = 42`, trying to
> overwrite the code from the C standard library.)
>
> Are Fortran processors allowed to place such "automatic data objects"
> like integer :: M(N,N) on the stack?

 From my reading, yes, they are allowed to do that. Local arrays can be 
put on the stack or the heap. Even the "allocatable" could be placed on 
the stack. But I am not a fortran expert.

Allocating on the stack has the problem that it is not possible to have 
a portable test whether there is enough space, hence the crash when it 
isn't. This is not specific to fortran. Some systems try to still detect 
such cases (like R), but it is not portable. There are OS-specific ways 
to increase the stack size limit, but that cannot be relied on with R, 
it would be rather too much asking R users to do that.

You might perhaps submit a bug report for flang-new, asking whether 
their heuristics for these cases are as intended, showing that they 
differ from gfortran.

You might get more help on mailing lists discussing Fortran language, 
specifically - this is not an R issue.

But in practice, yes, using "allocatable" should work much better for 
large arrays.

Best
Tomas


> The Fortran standard doesn't seem
> to give an answer to this question, but if you make your M allocatable,
> you won't have to worry about stack usage:
>
> subroutine dostuff(N,V)
>    implicit none
>
>    integer, intent(IN) :: N
>    integer, intent(INOUT) :: V(N*N)
>    integer, allocatable :: M(:,:) ! <-- here
>
>    allocate(M(N,N))               ! <-- and here
>    M = 42
>    V = RESHAPE(M, (/N*N/))
> end subroutine dostuff
>
> No leaks or crashes observed with these two changes and either
> compiler. The Fortran standard requires that local allocatable unsaved
> arrays (except for the function result) are deallocated at the end of
> procedures.
>