[Octave-bug-tracker] [bug #51950] Segfault during a sparse QR factorization

[Mike Miller]

Follow-up Comment #5, bug #51950 (project octave):

Marco - I agree, I think the matrix has to be the result of a QR
factorization.

In particular, qr may return an R matrix that is malformed, that has entries
that are out of bounds. For example


>> A = sprandn (5, 6, 0.1);
>> [~, R] = qr (A)
>> R
R =

Compressed Column Sparse (rows = 5, cols = 6, nnz = 3 [10%])

  (2, 2) -> -0.486301514709148
  (5, 5) -> -0.0638896858969156
  (6, 6) ->  0.290030632873829



Notice the size of the matrix R is 5×6, but it has an element at index (6,6).
This matrix crashes the subsequent qr(R) because it contains an element that
is out-of-bounds.

So the real bug here is that qr may return an R matrix with an element like
this shown above. I don't have the knowledge of the algorithm or the libraries
involved to dig further at the moment, someone want to look into this?

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