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Jed Brown committed 6c11f90

Mat SBAIJ: avoid invalid memory read in MatSOR_SBAIJ

In the back-solve, the software prefetching code computes the previous
line length and issues prefetch instructions before relaxing a row. For
the first row, the "previous row" does not exist so the indexing was
invalid. Now just make sure the index is non-negative.

Reported-by: Patrick Lacasse <patrick.m.lacasse@gmail.com>

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  • Parent commits 3b5ea25

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File src/mat/impls/sbaij/seq/relax.h

         nz = 0;
         for (i=m-1; i>=0; i--) {
           sum = b[i];
-          nz2 = ai[i] - ai[i-1] - 1;
+          nz2 = ai[i] - ai[PetscMax(i-1,0)] - 1; /* avoid referencing ai[-1], nonsense nz2 is okay on last iteration */
           PETSC_Prefetch(v-nz2-1,0,PETSC_PREFETCH_HINT_NTA);
           PETSC_Prefetch(vj-nz2-1,0,PETSC_PREFETCH_HINT_NTA);
           PetscSparseDenseMinusDot(sum,x,v,vj,nz);
         nz = 0;
         for (i=m-1; i>=0; i--) {
           sum = t[i];
-          nz2 = ai[i] - ai[i-1] - 1;
+          nz2 = ai[i] - ai[PetscMax(i-1,0)] - 1; /* avoid referencing ai[-1], nonsense nz2 is okay on last iteration */
           PETSC_Prefetch(v-nz2-1,0,PETSC_PREFETCH_HINT_NTA);
           PETSC_Prefetch(vj-nz2-1,0,PETSC_PREFETCH_HINT_NTA);
           PetscSparseDenseMinusDot(sum,x,v,vj,nz);