linux-kernel - Re: [PATCH 2/2] tracing: Stop fortify-string from warning in tracing_mark_raw

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Message-Id: <20251012123408.b50aad1b5abb8a63565067f5@kernel.org>
Date: Sun, 12 Oct 2025 12:34:08 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, stable@...r.kernel.org,
 syzbot+9a2ede1643175f350105@...kaller.appspotmail.com
Subject: Re: [PATCH 2/2] tracing: Stop fortify-string from warning in
 tracing_mark_raw_write()

On Fri, 10 Oct 2025 23:51:43 -0400
Steven Rostedt <rostedt@...nel.org> wrote:

> From: Steven Rostedt <rostedt@...dmis.org>
> 
> The way tracing_mark_raw_write() records its data is that it has the
> following structure:
> 
>   struct {
> 	struct trace_entry;
> 	int id;
> 	char dynamic_array[];
>   };
> 
> But memcpy(&entry->id, buf, size) triggers the following warning when the
> size is greater than the id:
> 
>  ------------[ cut here ]------------
>  memcpy: detected field-spanning write (size 6) of single field "&entry->id" at kernel/trace/trace.c:7458 (size 4)
>  WARNING: CPU: 7 PID: 995 at kernel/trace/trace.c:7458 write_raw_marker_to_buffer.isra.0+0x1f9/0x2e0
>  Modules linked in:
>  CPU: 7 UID: 0 PID: 995 Comm: bash Not tainted 6.17.0-test-00007-g60b82183e78a-dirty #211 PREEMPT(voluntary)
>  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
>  RIP: 0010:write_raw_marker_to_buffer.isra.0+0x1f9/0x2e0
>  Code: 04 00 75 a7 b9 04 00 00 00 48 89 de 48 89 04 24 48 c7 c2 e0 b1 d1 b2 48 c7 c7 40 b2 d1 b2 c6 05 2d 88 6a 04 01 e8 f7 e8 bd ff <0f> 0b 48 8b 04 24 e9 76 ff ff ff 49 8d 7c 24 04 49 8d 5c 24 08 48
>  RSP: 0018:ffff888104c3fc78 EFLAGS: 00010292
>  RAX: 0000000000000000 RBX: 0000000000000006 RCX: 0000000000000000
>  RDX: 0000000000000000 RSI: 1ffffffff6b363b4 RDI: 0000000000000001
>  RBP: ffff888100058a00 R08: ffffffffb041d459 R09: ffffed1020987f40
>  R10: 0000000000000007 R11: 0000000000000001 R12: ffff888100bb9010
>  R13: 0000000000000000 R14: 00000000000003e3 R15: ffff888134800000
>  FS:  00007fa61d286740(0000) GS:ffff888286cad000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000560d28d509f1 CR3: 00000001047a4006 CR4: 0000000000172ef0
>  Call Trace:
>   <TASK>
>   tracing_mark_raw_write+0x1fe/0x290
>   ? __pfx_tracing_mark_raw_write+0x10/0x10
>   ? security_file_permission+0x50/0xf0
>   ? rw_verify_area+0x6f/0x4b0
>   vfs_write+0x1d8/0xdd0
>   ? __pfx_vfs_write+0x10/0x10
>   ? __pfx_css_rstat_updated+0x10/0x10
>   ? count_memcg_events+0xd9/0x410
>   ? fdget_pos+0x53/0x5e0
>   ksys_write+0x182/0x200
>   ? __pfx_ksys_write+0x10/0x10
>   ? do_user_addr_fault+0x4af/0xa30
>   do_syscall_64+0x63/0x350
>   entry_SYSCALL_64_after_hwframe+0x76/0x7e
>  RIP: 0033:0x7fa61d318687
>  Code: 48 89 fa 4c 89 df e8 58 b3 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
>  RSP: 002b:00007ffd87fe0120 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
>  RAX: ffffffffffffffda RBX: 00007fa61d286740 RCX: 00007fa61d318687
>  RDX: 0000000000000006 RSI: 0000560d28d509f0 RDI: 0000000000000001
>  RBP: 0000560d28d509f0 R08: 0000000000000000 R09: 0000000000000000
>  R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000006
>  R13: 00007fa61d4715c0 R14: 00007fa61d46ee80 R15: 0000000000000000
>   </TASK>
>  ---[ end trace 0000000000000000 ]---
> 
> This is because fortify string sees that the size of entry->id is only 4
> bytes, but it is writing more than that. But this is OK as the
> dynamic_array is allocated to handle that copy.
> 
> Use a void pointer and get the offset via offsetof() to keep fortify
> string from warning about this copy.
> 
> Cc: stable@...r.kernel.org
> Fixes: 64cf7d058a00 ("tracing: Have trace_marker use per-cpu data to read user space")
> Reported-by: syzbot+9a2ede1643175f350105@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/68e973f5.050a0220.1186a4.0010.GAE@google.com/
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Ah, fixed already.

Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

> ---
>  kernel/trace/trace.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index bbb89206a891..27855fc9e0f2 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -7440,6 +7440,7 @@ static ssize_t write_raw_marker_to_buffer(struct trace_array *tr,
>  	struct raw_data_entry *entry;
>  	ssize_t written;
>  	size_t size;
> +	void *ptr;
>  
>  	size = sizeof(*entry) + cnt;
>  
> @@ -7455,7 +7456,10 @@ static ssize_t write_raw_marker_to_buffer(struct trace_array *tr,
>  		return -EBADF;
>  
>  	entry = ring_buffer_event_data(event);
> -	memcpy(&entry->id, buf, cnt);
> +	/* Do not let fortify-string warn copying to &entry->id */
> +	ptr = (void *)entry;
> +	ptr += offsetof(typeof(*entry), id);
> +	memcpy(ptr, buf, cnt);
>  	written = cnt;
>  
>  	__buffer_unlock_commit(buffer, event);
> -- 
> 2.51.0
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>