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Wed, 11 Nov 2026 10:03:30 +0000 (UTC) Received: from [HIDDEN] (Authenticated sender: rostedt@goodmis.org) by omf17.hostedemail.com (Postfix) with ESMTPA id D0EA719; Wed, 11 Nov 2026 10:03:27 +0000 (UTC) Date: Wed, 11 Nov 2026 05:03:22 -0500 From: Steven Rostedt To: LKML , Linux trace kernel Cc: Masami Hiramatsu , Mathieu Desnoyers Subject: [PATCH] tracing/probes: Zero out faulted dereferenced fields Message-ID: <20261111050322.49bbebfa@fedora> X-Mailer: Claws Mail 4.4.0 (GTK 3.24.52; x86_64-redhat-linux-gnu) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspamout05 X-Rspamd-Queue-Id: D0EA719 X-Stat-Signature: n971ugsmikxw4qo3d4gp7iy3a5w4d59b X-Session-Marker: 726F737465647440676F6F646D69732E6F7267 X-Session-ID: U2FsdGVkX187LOXOvnlUwba75FnJkjrRJvsphm/oBvM= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=goodmis.org; h=date:from:to:cc:subject:message-id:mime-version:content-type:content-transfer-encoding; s=dkim1; bh=nxPODO96ooeOFk6g2gU7/c+Ex4GmNCqkv9X+bjevByQ=; b=hv/aZhysHnZpNpVqmnLZPYq7QULWBWXyKL0StqiMcJ1S6N3DygwqS3WZwJa26BX+Yv1+r6k4SPU3neXom9e50kZtH+Vm1rnaHbC6Uz6cPTUMn+HXtEWoPHC17k1+4kH8+gewADI209ZwcuLngNdRbSdKBq91AggrBHhf7qFXp1o= X-HE-Tag: 1794391407-275707 X-HE-Meta: U2FsdGVkX188mOS6vxNQO29vO9nGP3G2ppzsqqxZfhscu/SwOszW7BamPGtoWWi6aLpUZboUh04A0+yym919beMxthXbS0mvuMxIqj/SpoJZcuI3fvnUmcSdx6OWRIOukG3lmM3PNX4Cgehepnbm6fJHCNZEQWZJm2cxgUABmz/rqAmEvqrpB22hUSQEVgqZsnpXiRlt+/BdWL4CQXAuVBcEqA6j33ZgejScM6UhM+feVSaPfR8Q+Ph+95uD14+oYDe6iTgScqZM1xRFGlG1eAcJ9nhKaavHRWCGeA+GRM4JjGOT3XLT7k1jHagKtc3LAMe0+xGCCNkcrNDf16nX1NW6pYsnYZH4xtzPKG8ZflYjhBQN76GqL089xT/NQVIYuNm8RFLAkPTgGR7lHdXuLA== From: Steven Rostedt While looking at tasks global descriptor tables, I discovered the following anomaly: ~# echo 't:sched sched_switch prev=prev mm=(task_struct)prev->mm ppgd=(task_struct)prev->mm->pgd' > /sys/kernel/tracing/dynamic_events The above creates a tprobe on the sched_switch tracepoint and records the previous task_struct pointer, it's mm_struct and the pgd the mm_struct points to. ~# trace-cmd start -e tracepoints:sched bash-1050 [000] d..3. 90.829478: sched: (__probestub_sched_switch+0x0/0x10) prev=0xffff88d30b623600 mm=0xffff88d30108c600 ppgd=0xffff88d33352b000 -0 [002] d..3. 90.836024: sched: (__probestub_sched_switch+0x0/0x10) prev=0xffff88d301870000 mm=0x0 ppgd=0x0 rcu_preempt-15 [002] d..3. 90.836031: sched: (__probestub_sched_switch+0x0/0x10) prev=0xffff88d3013ad100 mm=0x0 ppgd=0xffff88d33352b000 The first event has is a user task that has both a valid mm_struct and a pgd. The second event is a kernel task where the mm_struct is NULL and so is the dereferenced pgd pointer. The third event is where things looked strange. The mm_struct is NULL but the dereferenced pgd has a value! That is: NULL->pgd produced something other than zero. The reason for this is that the probe_mem_read() and probe_mem_read_user() do not write to the destination on failure. The process_fetch_insn_bottom() loop does not check the return value of the dereference (as it is the last one), and does not update the event. That leaves the value for that failed field to be whatever was in the ring buffer at that time. Have both probe_mem_read() and probe_mem_read_user() set the destination to zero on failure to read the address. Now all gdb are zero when mm is zero. Cc: stable@vger.kernel.org Fixes: 533059281ee59 ("tracing: probeevent: Introduce new argument fetching code") Signed-off-by: Steven Rostedt --- kernel/trace/trace_probe_kernel.h | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) diff --git a/kernel/trace/trace_probe_kernel.h b/kernel/trace/trace_probe_kernel.h index 4eb4acfa09ba..00542a162d30 100644 --- a/kernel/trace/trace_probe_kernel.h +++ b/kernel/trace/trace_probe_kernel.h @@ -102,18 +102,27 @@ static nokprobe_inline int probe_mem_read_user(void *dest, void *src, size_t size) { const void __user *uaddr = (__force const void __user *)src; + int ret; - return copy_from_user_nofault(dest, uaddr, size); + ret = copy_from_user_nofault(dest, uaddr, size); + if (ret < 0) + memset(dest, 0, size); + return ret; } static nokprobe_inline int probe_mem_read(void *dest, void *src, size_t size) { + int ret; + #ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE if ((unsigned long)src < TASK_SIZE) return probe_mem_read_user(dest, src, size); #endif - return copy_from_kernel_nofault(dest, src, size); + ret = copy_from_kernel_nofault(dest, src, size); + if (ret < 0) + memset(dest, 0, size); + return ret; } #endif /* __TRACE_PROBE_KERNEL_H_ */ -- 2.55.0