Re: [PATCH v2] xfsprogs: _{attr,data}_map_shared should take ILOCK_EXCL until iread_extents is completely done

[Dave Chinner <david@fromorbit.com>]

On Tue, Apr 11, 2023 at 03:53:38PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> While fuzzing the data fork extent count on a btree-format directory
> with xfs/375, I observed the following (excerpted) splat:
> 
> XFS: Assertion failed: xfs_isilocked(ip, XFS_ILOCK_EXCL), file: fs/xfs/libxfs/xfs_bmap.c, line: 1208
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 43192 at fs/xfs/xfs_message.c:104 assfail+0x46/0x4a [xfs]
> Call Trace:
>  <TASK>
>  xfs_iread_extents+0x1af/0x210 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xchk_dir_walk+0xb8/0x190 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xchk_parent_count_parent_dentries+0x41/0x80 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xchk_parent_validate+0x199/0x2e0 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xchk_parent+0xdf/0x130 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xfs_scrub_metadata+0x2b8/0x730 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xfs_scrubv_metadata+0x38b/0x4d0 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xfs_ioc_scrubv_metadata+0x111/0x160 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  xfs_file_ioctl+0x367/0xf50 [xfs 09f66509ece4938760fac7de64732a0cbd3e39cd]
>  __x64_sys_ioctl+0x82/0xa0
>  do_syscall_64+0x2b/0x80
>  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> 
> The cause of this is a race condition in xfs_ilock_data_map_shared,
> which performs an unlocked access to the data fork to guess which lock
> mode it needs:
> 
> Thread 0                          Thread 1
> 
> xfs_need_iread_extents
> <observe no iext tree>
> xfs_ilock(..., ILOCK_EXCL)
> xfs_iread_extents
> <observe no iext tree>
> <check ILOCK_EXCL>
> <load bmbt extents into iext>
> <notice iext size doesn't
>  match nextents>
>                                   xfs_need_iread_extents
>                                   <observe iext tree>
>                                   xfs_ilock(..., ILOCK_SHARED)
> <tear down iext tree>
> xfs_iunlock(..., ILOCK_EXCL)
>                                   xfs_iread_extents
>                                   <observe no iext tree>
>                                   <check ILOCK_EXCL>
>                                   *BOOM*
> 
> Fix this race by adding a flag to the xfs_ifork structure to indicate
> that we have not yet read in the extent records and changing the
> predicate to look at the flag state, not if_height.  The memory barrier
> ensures that the flag will not be set until the very end of the
> function.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
> Here's the userspace version, which steals heavily from the kernel but
> otherwise uses liburcu underneath.
> ---
>  include/atomic.h        |  100 +++++++++++++++++++++++++++++++++++++++++++++++
>  libxfs/libxfs_priv.h    |    3 -
>  libxfs/xfs_bmap.c       |    6 +++
>  libxfs/xfs_inode_fork.c |   16 +++++++-
>  libxfs/xfs_inode_fork.h |    6 ++-
>  5 files changed, 125 insertions(+), 6 deletions(-)
> 
> diff --git a/include/atomic.h b/include/atomic.h
> index 9c4aa5849ae..98889190bb0 100644
> --- a/include/atomic.h
> +++ b/include/atomic.h
> @@ -118,4 +118,104 @@ atomic64_set(atomic64_t *a, int64_t v)
>  
>  #endif /* HAVE_URCU_ATOMIC64 */
>  
> +#define __smp_mb()		cmm_smp_mb()
> +
> +/* from compiler_types.h */
> +/*
> + * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
> + *			       non-scalar types unchanged.
> + */
> +/*
> + * Prefer C11 _Generic for better compile-times and simpler code. Note: 'char'
> + * is not type-compatible with 'signed char', and we define a separate case.
> + */
> +#define __scalar_type_to_expr_cases(type)				\
> +		unsigned type:	(unsigned type)0,			\
> +		signed type:	(signed type)0
> +
> +#define __unqual_scalar_typeof(x) typeof(				\
> +		_Generic((x),						\
> +			 char:	(char)0,				\
> +			 __scalar_type_to_expr_cases(char),		\
> +			 __scalar_type_to_expr_cases(short),		\
> +			 __scalar_type_to_expr_cases(int),		\
> +			 __scalar_type_to_expr_cases(long),		\
> +			 __scalar_type_to_expr_cases(long long),	\
> +			 default: (x)))
> +
> +/* Is this type a native word size -- useful for atomic operations */
> +#define __native_word(t) \
> +	(sizeof(t) == sizeof(char) || sizeof(t) == sizeof(short) || \
> +	 sizeof(t) == sizeof(int) || sizeof(t) == sizeof(long))
> +
> +#define compiletime_assert(foo, str)	BUILD_BUG_ON(!(foo))
> +
> +#define compiletime_assert_atomic_type(t)				\
> +	compiletime_assert(__native_word(t),				\
> +		"Need native word sized stores/loads for atomicity.")
> +
> +/* from barrier.h */
> +#ifndef __smp_store_release
> +#define __smp_store_release(p, v)					\
> +do {									\
> +	compiletime_assert_atomic_type(*p);				\
> +	__smp_mb();							\
> +	WRITE_ONCE(*p, v);						\
> +} while (0)
> +#endif
> +
> +#ifndef __smp_load_acquire
> +#define __smp_load_acquire(p)						\
> +({									\
> +	__unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p);		\
> +	compiletime_assert_atomic_type(*p);				\
> +	__smp_mb();							\
> +	(typeof(*p))___p1;						\
> +})
> +#endif
> +
> +#ifndef smp_store_release
> +#define smp_store_release(p, v) __smp_store_release((p), (v))
> +#endif
> +
> +#ifndef smp_load_acquire
> +#define smp_load_acquire(p) __smp_load_acquire(p)
> +#endif
> +
> +/* from rwonce.h */
> +/*
> + * Yes, this permits 64-bit accesses on 32-bit architectures. These will
> + * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
> + * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
> + * (e.g. a virtual address) and a strong prevailing wind.
> + */
> +#define compiletime_assert_rwonce_type(t)					\
> +	compiletime_assert(__native_word(t) || sizeof(t) == sizeof(long long),	\
> +		"Unsupported access size for {READ,WRITE}_ONCE().")
> +
> +/*
> + * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
> + * atomicity. Note that this may result in tears!
> + */
> +#ifndef __READ_ONCE
> +#define __READ_ONCE(x)	(*(const volatile __unqual_scalar_typeof(x) *)&(x))
> +#endif
> +
> +#define READ_ONCE(x)							\
> +({									\
> +	compiletime_assert_rwonce_type(x);				\
> +	__READ_ONCE(x);							\
> +})
> +
> +#define __WRITE_ONCE(x, val)						\
> +do {									\
> +	*(volatile typeof(x) *)&(x) = (val);				\
> +} while (0)
> +
> +#define WRITE_ONCE(x, val)						\
> +do {									\
> +	compiletime_assert_rwonce_type(x);				\
> +	__WRITE_ONCE(x, val);						\
> +} while (0)
> +
>  #endif /* __ATOMIC_H__ */

I'd put READ/WRITE_ONCE above the barrier stuff as
__smp_store_release/__smp_load_acquire use it, but other than that
it looks OK.

> diff --git a/libxfs/libxfs_priv.h b/libxfs/libxfs_priv.h
> index e5f37df28c0..5fdfeeea060 100644
> --- a/libxfs/libxfs_priv.h
> +++ b/libxfs/libxfs_priv.h
> @@ -222,9 +222,6 @@ static inline bool WARN_ON(bool expr) {
>  #define percpu_counter_read_positive(x)	((*x) > 0 ? (*x) : 0)
>  #define percpu_counter_sum(x)		(*x)
>  
> -#define READ_ONCE(x)			(x)
> -#define WRITE_ONCE(x, val)		((x) = (val))

Yay! One less nasty hack for the userspace wrappers.

Overall looks good to me.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
-- 
Dave Chinner
david@fromorbit.com