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SubjectRe: [PATCH] x86/mm: Do not verify W^X at boot up
On Mon, 24 Oct 2022 11:19:31 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:


> in text_poke_bp(). And that, in turn, is because PeterZ ended up
> special-casing this all in commit 768ae4406a5c ("x86/ftrace: Use
> text_poke()")
>
> Maybe we should just strive to get rid of all these SYSTEM_BOOTING
> special cases, instead of adding yet another a new one.
>
> There's presumably "it slows down boot" reason to avoid the full
> text_poke_bp() dance, but do we really care for the "ftrace=function"
> case?
>

It's not just speed up at boot up. It's because text_poke doesn't work at
early boot up when function tracing is enabled. If I remove the
SYSTEM_BOOTING checks in text_poke() this happens:

[ 0.963966] ftrace: allocating 47021 entries in 184 pages
[ 0.969376] ftrace section at ffffffff89ef54c0 sorted properly
[ 0.982126] ftrace: allocated 184 pages with 4 groups
[ 1.009779] Starting tracer 'function'
[ 1.013753] BUG: kernel NULL pointer dereference, address: 0000000000000048
[ 1.020516] #PF: supervisor read access in kernel mode
[ 1.025616] #PF: error_code(0x0000) - not-present page
[ 1.030718] PGD 0 P4D 0
[ 1.033224] Oops: 0000 [#1] PREEMPT SMP PTI
[ 1.037373] CPU: 0 PID: 0 Comm: swapper Not tainted 6.1.0-rc1-test+ #496
[ 1.044031] Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v03.03 07/14/2016
[ 1.052934] RIP: 0010:walk_to_pmd+0x17/0x130
[ 1.057172] Code: 1f ff ff ff 0f 0b e9 ed fe ff ff 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f0 41 55 48 c1 e8 24 41 54 25 f8 0f 00 00 55 53 <48> 03 47 48 0f 84 c4 00 00 00 49 89 fc 48 8b 38 48 89 f3 48 89 c5
[ 1.075846] RSP: 0000:ffffffff89603cd8 EFLAGS: 00010046
[ 1.081033] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffd0f980000000
[ 1.088121] RDX: ffffffff89603d68 RSI: 0000000000000000 RDI: 0000000000000000
[ 1.095213] RBP: 0000000000000000 R08: ffffffff8a017cf0 R09: ffffffff8a017cf1
[ 1.102302] R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff89603d68
[ 1.109389] R13: 000000000000031e R14: 000000000000031f R15: 8000000000000063
[ 1.116479] FS: 0000000000000000(0000) GS:ffffa068daa00000(0000) knlGS:0000000000000000
[ 1.124516] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1.130220] CR2: 0000000000000048 CR3: 000000000760a001 CR4: 00000000000606f0
[ 1.137309] Call Trace:
[ 1.139732] <TASK>
[ 1.141805] __get_locked_pte+0x1b/0x120
[ 1.145694] ? ftrace_caller_op_ptr+0x17/0x17
[ 1.150016] __text_poke+0xf8/0x540
[ 1.153474] ? patch_retpoline+0x170/0x170
[ 1.157536] ? text_poke_loc_init+0x5b/0x170
[ 1.161773] text_poke_bp_batch+0x86/0x290
[ 1.165835] ? cache_mod+0x280/0x280
[ 1.169378] text_poke_bp+0x3a/0x60
[ 1.172836] ftrace_update_ftrace_func+0x3e/0x80
[ 1.177416] ftrace_modify_all_code+0x79/0x160
[ 1.181827] ftrace_startup+0xbd/0x150
[ 1.185544] ? ftrace_stacktrace_count+0xc0/0xc0
[ 1.190128] register_ftrace_function_nolock+0x20/0x60
[ 1.195227] register_ftrace_function+0xc7/0x130
[ 1.199807] ? ftrace_stacktrace_count+0xc0/0xc0
[ 1.204390] function_trace_init+0x71/0xd0
[ 1.208454] tracing_set_tracer+0x172/0x280
[ 1.212603] register_tracer+0x1e0/0x205
[ 1.216495] tracer_alloc_buffers.isra.0+0x1f8/0x2fe
[ 1.221421] start_kernel+0x21f/0x4e8
[ 1.225051] ? x86_64_start_kernel+0x60/0x78
[ 1.229288] secondary_startup_64_no_verify+0xd3/0xdb
[ 1.234301] </TASK>
[ 1.236459] Modules linked in:
[ 1.239484] CR2: 0000000000000048
[ 1.242769] ---[ end trace 0000000000000000 ]---

Interrupts haven't been enabled yet, so things are still rather fragile at
this point of start up.

-- Steve

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