Re: Corrupted bp->b_lblkno on bread() // Life-cycle of a buf obj?

> There is something strange in the sentence.  First you claim that
> b_blkno == b_lblkno, then you claim thant b_lbkno changes from 0 to some
> random value.

Apologies for the confusing phrasing. What I meant by this is that
pre-calling VOP_STRATEGY blkno and lblkno are the same (both are 0 in this
particular case), which implies there needs to be a bmap call.

> And this smells like an KBI (Kernel Binary Interface) issue, since
DEBUG_LOCKS
> changes the layout of the struct lock, which is embedded into struct buf
> with which you have problems.

> How do you build your fs code? As a module?  If yes, you must use the same
> set of opt_*.h headers as used for the kernel build.

I think this might be it, I am building it as a kmod and hadn't taken the
changed struct into account. Will try including these headers. Was starting
to see similar behaviour creep up in a different code path as well. Thanks
for the help!

On Thu, 19 Jun 2025 at 14:42, Konstantin Belousov <kostikbel@gmail.com>
wrote:

> On Tue, Jun 17, 2025 at 11:07:49PM -0400, Sanchit Sahay wrote:
> > I'm working on porting a filesystem to FreeBSD, and am running into an
> > issue that I'm having difficulty debugging. Any help would be
> appreciated.
> >
> > When calling bread() with an blkno=lblkno, by the time the flow of the
> > control reaches the vop_strategy function, the value of lblkno changes
> from
> > 0 to a seemingly random value.
> There is something strange in the sentence.  First you claim that
> b_blkno == b_lblkno, then you claim thant b_lbkno changes from 0 to some
> random value.
>
> So, is it 0 or b_blkno?
>
> >
> > Having inspected this with gdb,
> >
> > On frame 9:
> >
> > #9  0xffff0000c3e72930 in hfs_strategy ()
> > 1488            kdb_enter("lblk random", "lblk random");
> >
> > *(kgdb) p ap->a_bp->b_lblkno$10 = -281474971149872*
> >
> > On frame 10:
> >
> > #10 0xffff0000009387b0 in VOP_STRATEGY_APV () at vnode_if.c:2423
> > 2423                    rc = vop->vop_strategy(a);
> >
> > *(kgdb) p a->a_bp->b_lblkno$11 = 0*
> And the same pattern occurs there.
>
> >
> > This flow is triggered when calling bread() like so:
> >
> > retval = bread(vp, blockNum, block->blockSize, NOCRED, &bp);
> >
> > The stack trace is:
> >
> > #9  0xffff0000c3e72930 in hfs_strategy (ap=0xffff00009bbd1058)
> > #10 0xffff0000009387b0 in VOP_STRATEGY_APV (
> > #11 0xffff00000054bbcc in VOP_STRATEGY (vp=0xffff000000a08fc5,
> > #12 bufstrategy (bo=<optimized out>, bp=0xffff0000404990c8)
> > #13 0xffff00000054d6f0 in bstrategy (bp=0xffff0000404990c8)
> > #14 breadn_flags
> >
> > There seems to be no code run between these two stacks, the a_bp in both
> > these frames points to the same memory address. No other fields are
> > modified between these two frames.
> >
> > Because of this seemingly random lblkno value, VOP_BMAP is not triggered,
> > and the read returns arbitrary results.
> >
> > This issue only occurs when I have the kernel compiled with these
> > additional flags (as suggested by the handbook for debugging deadlocks):
> >
> > options INVARIANTS
> > options INVARIANT_SUPPORT
> > options WITNESS
> > options WITNESS_SKIPSPIN
> > options DEBUG_LOCKS
> > options DEBUG_VFS_LOCKS
> > options DIAGNOSTIC
> >
> > Without these flags enabled, this lblkno corruption does not take place,
> > and the bread returns a valid read. I don't see any conditional code that
> > these flags enable which would cause such an issue.
> And this smells like an KBI (Kernel Binary Interface) issue, since
> DEBUG_LOCKS
> changes the layout of the struct lock, which is embedded into struct buf
> with which you have problems.
>
> How do you build your fs code? As a module?  If yes, you must use the same
> set of opt_*.h headers as used for the kernel build.
>
> >
> > Any tips on how to investigate this further would be greatly appreciated,
> > or if I am missing something about the lifecycle of the buffer object
> that
> > might cause it to "reset" certain fields.
> >
> > Thanks
> > Sanchit Sahay
>