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In commit 15d9da3f818c ("binder: use bitmap for faster descriptor
lookup"), it was incorrectly assumed that references to the context
manager node should always get descriptor zero assigned to them.
However, if the context manager dies and a new process takes its place,
then assigning descriptor zero to the new context manager might lead to
collisions, as there could still be references to the older node. This
issue was reported by syzbot with the following trace:
kernel BUG at drivers/android/binder.c:1173!
Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
Modules linked in:
CPU: 1 PID: 447 Comm: binder-util Not tainted 6.10.0-rc6-00348-g31643d84b8c3 #10
Hardware name: linux,dummy-virt (DT)
pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : binder_inc_ref_for_node+0x500/0x544
lr : binder_inc_ref_for_node+0x1e4/0x544
sp : ffff80008112b940
x29: ffff80008112b940 x28: ffff0e0e40310780 x27: 0000000000000000
x26: 0000000000000001 x25: ffff0e0e40310738 x24: ffff0e0e4089ba34
x23: ffff0e0e40310b00 x22: ffff80008112bb50 x21: ffffaf7b8f246970
x20: ffffaf7b8f773f08 x19: ffff0e0e4089b800 x18: 0000000000000000
x17: 0000000000000000 x16: 0000000000000000 x15: 000000002de4aa60
x14: 0000000000000000 x13: 2de4acf000000000 x12: 0000000000000020
x11: 0000000000000018 x10: 0000000000000020 x9 : ffffaf7b90601000
x8 : ffff0e0e48739140 x7 : 0000000000000000 x6 : 000000000000003f
x5 : ffff0e0e40310b28 x4 : 0000000000000000 x3 : ffff0e0e40310720
x2 : ffff0e0e40310728 x1 : 0000000000000000 x0 : ffff0e0e40310710
Call trace:
binder_inc_ref_for_node+0x500/0x544
binder_transaction+0xf68/0x2620
binder_thread_write+0x5bc/0x139c
binder_ioctl+0xef4/0x10c8
[...]
This patch adds back the previous behavior of assigning the next
non-zero descriptor if references to previous context managers still
exist. It amends both strategies, the newer dbitmap code and also the
legacy slow_desc_lookup_olocked(), by allowing them to start looking
for available descriptors at a given offset.
Fixes: 15d9da3f818c ("binder: use bitmap for faster descriptor lookup")
Cc: stable@vger.kernel.org
Reported-and-tested-by: syzbot+3dae065ca76952a67257@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000c1c0a0061d1e6979@google.com/
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20240722150512.4192473-1-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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When creating new binder references, the driver assigns a descriptor id
that is shared with userspace. Regrettably, the driver needs to keep the
descriptors small enough to accommodate userspace potentially using them
as Vector indexes. Currently, the driver performs a linear search on the
rb-tree of references to find the smallest available descriptor id. This
approach, however, scales poorly as the number of references grows.
This patch introduces the usage of bitmaps to boost the performance of
descriptor assignments. This optimization results in notable performance
gains, particularly in processes with a large number of references. The
following benchmark with 100,000 references showcases the difference in
latency between the dbitmap implementation and the legacy approach:
[ 587.145098] get_ref_desc_olocked: 15us (dbitmap on)
[ 602.788623] get_ref_desc_olocked: 47343us (dbitmap off)
Note the bitmap size is dynamically adjusted in line with the number of
references, ensuring efficient memory usage. In cases where growing the
bitmap is not possible, the driver falls back to the slow legacy method.
A previous attempt to solve this issue was proposed in [1]. However,
such method involved adding new ioctls which isn't great, plus older
userspace code would not have benefited from the optimizations either.
Link: https://lore.kernel.org/all/20240417191418.1341988-1-cmllamas@google.com/ [1]
Cc: Tim Murray <timmurray@google.com>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Martijn Coenen <maco@android.com>
Cc: Todd Kjos <tkjos@android.com>
Cc: John Stultz <jstultz@google.com>
Cc: Steven Moreland <smoreland@google.com>
Suggested-by: Nick Chen <chenjia3@oppo.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20240612042535.1556708-1-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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