[PATCH 0/1] Fix random.c UB on int overflow

[Lukas Herbolt <lukas@herbolt.com>]

While building xfstests with default rpm CFLAGS/LDFLAGS, I noticed
the generic/007 fails with:

    --- /opt/xfstests-dev/tests/generic/007.out	2026-10-06 14:08:02.556220624 +0200
    +++ /opt/xfstests-dev/results//generic/007.out.bad	2026-10-06 14:40:01.667593670 +0200
    @@ -14,9 +14,9 @@
     .........................................................................
     .........................................................................
     ....................................................
    -creates:  18736 OK,  18802 EEXIST  ( 37538 total, 50% EEXIST)
    -removes:  18675 OK,  19927 ENOENT  ( 38602 total, 51% ENOENT)
    -lookups:  12000 OK,  11860 ENOENT  ( 23860 total, 49% ENOENT)
    -total  :  49411 OK,  50589 w/error (100000 total, 50% w/error)
    +creates:  18745 OK,  19086 EEXIST  ( 37831 total, 50% EEXIST)
    +removes:  18685 OK,  19864 ENOENT  ( 38549 total, 51% ENOENT)
    +lookups:  12065 OK,  11555 ENOENT  ( 23620 total, 48% ENOENT)
    +total  :  49495 OK,  50505 w/error (100000 total, 50% w/error)
     
    -cleanup:     61 removes
    +cleanup:     60 removes

This happens because because the lib/random.c has optimized out the negative
branch check as overflow on signed int is undefined behavior.

    if (it <= 0)	
        it = (it + it) ^ MASK;
    else
        it = it + it;

Changing the if-else to single if based on the original `it` value prevents the
compiler from optimizing out the code.

    int apply_mask = (it <= 0);
    it = (unsigned)it << 1;
    if (apply_mask)
        it ^= MASK;

Steps to reproduce: 
    dnf install redhat-rpm-config -y
    ./configure CFLAGS="$(rpm --eval '%{optflags}')" \
        LDFLAGS="$(rpm --eval '%{__global_ldflags}')"
    ./check generic/007

The random.c is used in:

    ext4/052
    generic/007
    generic/010
    generic/011
    generic/014
    generic/094
    generic/225
    generic/311
    generic/676
    xfs/008
    xfs/188

and I did not noticed any regression with this change and the only one
failing with the original implementation is the generic/007. 

Lukas Herbolt (1):
  lib/random.c fix undefined behavior on signed integer overflow.

 lib/random.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

-- 
2.55.0