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charset="utf-8" Content-ID: Content-Transfer-Encoding: quoted-printable Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: RE: [PATCH] ceph: Fix potential undefined behavior in crush_ln() with GCC 11.1.0 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1117,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-09-22_01,2025-09-22_01,2025-03-28_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 phishscore=0 priorityscore=1501 malwarescore=0 adultscore=0 clxscore=1015 impostorscore=0 spamscore=0 suspectscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2507300000 definitions=main-2509190224 On Sat, 2025-09-20 at 20:06 +0800, =E9=99=88=E5=8D=8E=E6=98=AD=EF=BC=88Lyic= an=EF=BC=89 wrote: > > 2025=E5=B9=B49=E6=9C=8820=E6=97=A5 02:51=EF=BC=8CViacheslav Dubeyko =E5=86=99=E9=81=93=EF=BC=9A > >=20 > > On Fri, 2025-09-19 at 10:34 +0800, =E9=99=88=E5=8D=8E=E6=98=AD=EF=BC=88= Lyican=EF=BC=89 wrote: > > > > 2025=E5=B9=B49=E6=9C=8819=E6=97=A5 02:07=EF=BC=8CViacheslav Dubeyko= =E5=86=99=E9=81=93=EF=BC=9A > > > >=20 > >=20 > > > > I still have the same issue with the new patch. Your patch is trying to= modify > > the line 262. However, we have comments on this line [1]: > >=20 > > 260 /* > > 261 * figure out number of bits we need to shift and > > 262 * do it in one step instead of iteratively > > 263 */ > > 264 if (!(x & 0x18000)) { > > 265 int bits =3D __builtin_clz(x & 0x1FFFF) - 16; > > 266 x <<=3D bits; > > 267 iexpon =3D 15 - bits; > > 268 } > >=20 > > Thanks, > > Slava. > >=20 > > [1] > > https://elixir.bootlin.com/linux/v6.17-rc6/source/net/ceph/crush/mapper= .c#L262 =20 > Hi Slava, >=20 > Thank you for your patience with this patch. I want to clarify the confus= ion about the line numbering. >=20 > The patch header "@@ -262,7 +262,7 @@" was automatically generated by git= format-patch - I did not manually specify line 262. This is how git diff f= ormat works: it shows context lines starting from line 262, but the actual = code modification is on line 265 where the `__builtin_clz()` call is locate= d (exactly as you referenced in [1]). >=20 > To be absolutely clear: > - I am NOT trying to modify line 262 (which contains comments) > - I AM modifying line 265: `int bits =3D __builtin_clz(x & 0x1FFFF) - 16;` > - The "@@ -262,7 +262,7 @@" header is git's standard way of providing con= text > - Git automatically chooses how many context lines to show and where to s= tart them >=20 > The patch content clearly shows the actual change: > ```diff > - int bits =3D __builtin_clz(x & 0x1FFFF) - 16; > + int bits =3D (x & 0x1FFFF) ? __builtin_clz(x & 0x1FFFF) - 16 : 16; > ``` >=20 > This line-by-line diff shows exactly what gets modified - line 265 in the= official kernel source. >=20 > Here is the git-generated patch: >=20 > --- >=20 > From ac3a55a6a18761d613971ef6f78fa39e6d7d2172 Mon Sep 17 00:00:00 2001 > From: Huazhao Chen > Date: Sat, 20 Sep 2025 19:42:54 +0800 > Subject: [PATCH] ceph: Fix potential undefined behavior in crush_ln() wit= h GCC > =C2=A011.1.0 >=20 > When x & 0x1FFFF equals zero, __builtin_clz() is called with a zero > argument, which results in undefined behavior. This can happen during > ceph's consistent hashing calculations and may lead to incorrect > placement group mappings. >=20 > Fix by checking if the masked value is non-zero before calling > __builtin_clz(). If the masked value is zero, use the expected result > of 16 directly. >=20 > Signed-off-by: Huazhao Chen > --- > =C2=A0net/ceph/crush/mapper.c | 2 +- > =C2=A01 file changed, 1 insertion(+), 1 deletion(-) >=20 > diff --git a/net/ceph/crush/mapper.c b/net/ceph/crush/mapper.c > index 3a5bd1cd1..000f7a633 100644 > --- a/net/ceph/crush/mapper.c > +++ b/net/ceph/crush/mapper.c > @@ -262,7 +262,7 @@ static __u64 crush_ln(unsigned int xin) > =C2=A0=C2=A0* do it in one step instead of iteratively > =C2=A0=C2=A0*/ > =C2=A0=C2=A0if (!(x & 0x18000)) { > - int bits =3D __builtin_clz(x & 0x1FFFF) - 16; > + int bits =3D (x & 0x1FFFF) ? __builtin_clz(x & 0x1FFFF) - 16 : 16; > =C2=A0=C2=A0x <<=3D bits; > =C2=A0=C2=A0iexpon =3D 15 - bits; > =C2=A0=C2=A0} I completely lost myself in the multiple versions of the patch. Could you p= lease send one formal patch only that I can successfully apply? Thanks, Slava.