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From:   Francis Laniel <flaniel@linux.microsoft.com>
To:     linux-trace-devel@vger.kernel.org
Cc:     James Morse <james.morse@arm.com>,
        Daniel Kiss <daniel.kiss@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Mark Brown <broonie@kernel.org>,
        Peter Collingbourne <pcc@google.com>,
        Kees Cook <keescook@chromium.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-riscv@lists.infradead.org
Subject: Re: [PATCH v2 0/1] Remove forget_syscall() from start_thread_common()
Date:   Thu, 23 Jun 2022 16:09:01 +0200
Message-ID: <2635206.mvXUDI8C0e@pwmachine>
Organization: Microsoft
In-Reply-To: <20220608162447.666494-1-flaniel@linux.microsoft.com>
References: <20220608162447.666494-1-flaniel@linux.microsoft.com>
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Hi.

Le mercredi 8 juin 2022, 18:24:45 CEST Francis Laniel a =E9crit :
> Hi.
>=20
>=20
> First, I hope you are fine and the same for your relatives.
>=20
> With this contribution, I enabled using syscalls:sys_exit_execve and
> syscalls:sys_exit_execveat as tracepoints on arm64.
> Indeed, before this contribution, the above tracepoint would not print th=
eir
> information as syscall number was set to -1 by calling forget_syscall().
>=20
> I tested it by compiling a kernel for arm64 and running it within a VM:
> # Perf was compiled with linux kernel source.
> root@vm-arm64:~# perf record -ag -e 'syscalls:sys_exit_execve' -e
> 'syscalls:sys_enter_execve' & [1] 263
> root@vm-arm64:~# ls
> perf.data  share
> root@vm-arm64:~# fg
> perf record -ag -e 'syscalls:sys_exit_execve' -e 'syscalls:sys_enter_exec=
ve'
> ^C[ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.061 MB perf.data (2 samples) ]
> root@vm-arm64:~# perf script
> bash   264 [000]    66.220187: syscalls:sys_enter_execve: filename:
> 0xaaab05d9d ...
> # Below line does not appear without this patch.
> ls   264 [000]    66.226848:  syscalls:sys_exit_execve: 0x0
> ...
>=20
> Forgetting the syscall number before starting a new thread was confirmed =
to
> be a bug [1].
> Particularly, the following architectures do not forget the syscall number
> before starting a new thread:
> * arm (32 bits) EABI: start_thread() sets r7 to previous r7 for ELF FDPIC=
=20
> and to 0 for other binfmts [2].
> * arm (32 bits) OABI: syscall number is set to -1 if
> ptrace_report_syscall_entry() failed [3].
> * mips: start_thread() does not modify current_thread_info->syscall which=
 is
> taken directly from v0 [4, 5].
> * riscv: start_thread() does not modify a7 [6].
> * x86_64: start_thread_common() does not touch orig_ax which seems to
> contain the syscall number [7].
>=20
> If you see any way to improve this contribution, feel free to share!
>=20
> Change since:
> v1:
>  * Remove call to forget_syscall() and store previous syscall number in
>  regs->syscallno unconditionnaly.
>=20
> Francis Laniel (1):
>   arm64: Do not forget syscall when starting a new thread.
>=20
>  arch/arm64/include/asm/processor.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>=20
>=20
> Best regards and thank you in advance.
> ---
> [1] https://lore.kernel.org/linux-arm-kernel/YoT1iLPEbteRTQGZ@arm.com/
> [2] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/arm/include/as=
m/
> processor.h#L52
> [3] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/arm/kernel/
> ptrace.c#L847
> [4] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/mips/kernel/
> process.c#L52
> [5] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/mips/kernel/
> scall64-n64.S#L85
> [6] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/riscv/kernel/
> process.c#L87
> [7] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/x86/kernel/
> process_64.c#L505

Can someone please take a quick look at this patch?


Best regards and thank you in advance.