在 2021/4/8 19:20, Jisheng Zhang 写道: > On Tue, 30 Mar 2021 16:18:48 +0800 > Liao Chang <liaocha...@huawei.com> wrote: > > >> >> The execution of sys_read end up hitting a BUG_ON() in __find_get_block >> after installing kprobe at sys_read, the BUG message like the following: >> >> [ 65.708663] ------------[ cut here ]------------ >> [ 65.709987] kernel BUG at fs/buffer.c:1251! >> [ 65.711283] Kernel BUG [#1] >> [ 65.712032] Modules linked in: >> [ 65.712925] CPU: 0 PID: 51 Comm: sh Not tainted 5.12.0-rc4 #1 >> [ 65.714407] Hardware name: riscv-virtio,qemu (DT) >> [ 65.715696] epc : __find_get_block+0x218/0x2c8 >> [ 65.716835] ra : __getblk_gfp+0x1c/0x4a >> [ 65.717831] epc : ffffffe00019f11e ra : ffffffe00019f56a sp : >> ffffffe002437930 >> [ 65.719553] gp : ffffffe000f06030 tp : ffffffe0015abc00 t0 : >> ffffffe00191e038 >> [ 65.721290] t1 : ffffffe00191e038 t2 : 000000000000000a s0 : >> ffffffe002437960 >> [ 65.723051] s1 : ffffffe00160ad00 a0 : ffffffe00160ad00 a1 : >> 000000000000012a >> [ 65.724772] a2 : 0000000000000400 a3 : 0000000000000008 a4 : >> 0000000000000040 >> [ 65.726545] a5 : 0000000000000000 a6 : ffffffe00191e000 a7 : >> 0000000000000000 >> [ 65.728308] s2 : 000000000000012a s3 : 0000000000000400 s4 : >> 0000000000000008 >> [ 65.730049] s5 : 000000000000006c s6 : ffffffe00240f800 s7 : >> ffffffe000f080a8 >> [ 65.731802] s8 : 0000000000000001 s9 : 000000000000012a s10: >> 0000000000000008 >> [ 65.733516] s11: 0000000000000008 t3 : 00000000000003ff t4 : >> 000000000000000f >> [ 65.734434] t5 : 00000000000003ff t6 : 0000000000040000 >> [ 65.734613] status: 0000000000000100 badaddr: 0000000000000000 cause: >> 0000000000000003 >> [ 65.734901] Call Trace: >> [ 65.735076] [<ffffffe00019f11e>] __find_get_block+0x218/0x2c8 >> [ 65.735417] [<ffffffe00020017a>] __ext4_get_inode_loc+0xb2/0x2f6 >> [ 65.735618] [<ffffffe000201b6c>] ext4_get_inode_loc+0x3a/0x8a >> [ 65.735802] [<ffffffe000203380>] ext4_reserve_inode_write+0x2e/0x8c >> [ 65.735999] [<ffffffe00020357a>] __ext4_mark_inode_dirty+0x4c/0x18e >> [ 65.736208] [<ffffffe000206bb0>] ext4_dirty_inode+0x46/0x66 >> [ 65.736387] [<ffffffe000192914>] __mark_inode_dirty+0x12c/0x3da >> [ 65.736576] [<ffffffe000180dd2>] touch_atime+0x146/0x150 >> [ 65.736748] [<ffffffe00010d762>] filemap_read+0x234/0x246 >> [ 65.736920] [<ffffffe00010d834>] generic_file_read_iter+0xc0/0x114 >> [ 65.737114] [<ffffffe0001f5d7a>] ext4_file_read_iter+0x42/0xea >> [ 65.737310] [<ffffffe000163f2c>] new_sync_read+0xe2/0x15a >> [ 65.737483] [<ffffffe000165814>] vfs_read+0xca/0xf2 >> [ 65.737641] [<ffffffe000165bae>] ksys_read+0x5e/0xc8 >> [ 65.737816] [<ffffffe000165c26>] sys_read+0xe/0x16 >> [ 65.737973] [<ffffffe000003972>] ret_from_syscall+0x0/0x2 >> [ 65.738858] ---[ end trace fe93f985456c935d ]--- >> >> A simple reproducer looks like: >> echo 'p:myprobe sys_read fd=%a0 buf=%a1 count=%a2' > >> /sys/kernel/debug/tracing/kprobe_events >> echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable >> cat /sys/kernel/debug/tracing/trace >> > > I can't reproduce the BUG_ON with the above step, I may miss something. > My test platform versions Kernel: 0d02ec6b3136 Linux 5.12-rc4 QEMU: fdd76fecdd Update version for v5.0.0 release
>> Here's what happens to hit that BUG_ON(): >> >> 1) After installing kprobe at entry of sys_read, the first instruction >> is replaced by 'ebreak' instruction on riscv64 platform. >> >> 2) Once kernel reach the 'ebreak' instruction at the entry of sys_read, >> it trap into the riscv breakpoint handler, where it do something to >> setup for coming single-step of origin instruction, including backup >> the 'sstatus' in pt_regs, followed by disable interrupt during single >> stepping via clear 'SIE' bit of 'sstatus' in pt_regs. >> >> 3) Then kernel restore to the instruction slot contains two instructions, >> one is original instruction at entry of sys_read, the other is 'ebreak'. >> Here it trigger a 'Instruction page fault' exception (value at 'scause' >> is '0xc'), if PF is not filled into PageTabe for that slot yet. >> >> 4) Again kernel trap into page fault exception handler, where it choose >> different policy according to the state of running kprobe. Because >> afte 2) the state is KPROBE_HIT_SS, so kernel reset the current kprobe >> and 'pc' points back to the probe address. >> >> 5) Because 'epc' point back to 'ebreak' instrution at sys_read probe, >> kernel trap into breakpoint handler again, and repeat the operations >> at 2), however 'sstatus' without 'SIE' is keep at 4), it cause the >> real 'sstatus' saved at 2) is overwritten by the one withou 'SIE'. > > Is kprobe_single_step_handler() handled firstly this time? thus we won't > enter kprobe_breakpoint_handler(). > No,because this time kcb->ss.ctx.match_addr points to the single-step slot,but instruction_pointer(regs) points to the first instruction of sys_read('ebreak') so the condition is not token eventually, then we enter kprobe_breakpoint_handler(). bool __kprobes kprobe_single_step_handler(struct pt_regs *regs) { struct kprobe_ctlblk *kcb = get_kprobe_ctlblk(); if ((kcb->ss_ctx.ss_pending) && (kcb->ss_ctx.match_addr == instruction_pointer(regs))) { >> >> 6) When kernel cross the probe the 'sstatus' CSR restore with value >> without 'SIE', and reach __find_get_block where it requires the >> interrupt must be enabled. >> >> Fix this is very trivial, just restore the value of 'sstatus' in pt_regs >> with backup one at 2) when the instruction being single stepped cause a >> page fault. > > > PS: I'd suggest improve the commit msg. To be honest, I can't exactly > get all the meanings of above 1) ~ 6) > . I will reorganize commit msg, Thanks for review. > BR, Liao Chang