Re: [PATCH] riscv/ptrace: add new regset to get original a0 register

From: "Dmitry V. Levin" <ldv@strace.io>
To: Charlie Jenkins <charlie@rivosinc.com>
Cc: "Celeste Liu" <uwu@coelacanthus.name>,
	"Oleg Nesterov" <oleg@redhat.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Eric Biederman" <ebiederm@xmission.com>,
	"Kees Cook" <kees@kernel.org>, "Alexandre Ghiti" <alex@ghiti.fr>,
	"Andrea Bolognani" <abologna@redhat.com>,
	"Björn Töpel" <bjorn@kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ron Economos" <re@w6rz.net>,
	"Felix Yan" <felixonmars@archlinux.org>,
	"Ruizhe Pan" <c141028@gmail.com>,
	"Shiqi Zhang" <shiqi@isrc.iscas.ac.cn>,
	"Guo Ren" <guoren@kernel.org>, "Yao Zi" <ziyao@disroot.org>,
	"Han Gao" <gaohan@iscas.ac.cn>,
	"Quan Zhou" <zhouquan@iscas.ac.cn>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH] riscv/ptrace: add new regset to get original a0 register
Date: Tue, 3 Dec 2024 14:19:48 +0200	[thread overview]
Message-ID: <20241203121948.GA18179@strace.io> (raw)
In-Reply-To: <Z06ZAP-_4J4-6aK_@ghost>

On Mon, Dec 02, 2024 at 09:37:04PM -0800, Charlie Jenkins wrote:
[...]
> +static void ptrace_test(int opt, int *result)
> +{
> +	int status;
> +	pid_t pid;
> +	struct user_regs_struct regs;
> +	struct iovec iov = {
> +		.iov_base = &regs,
> +		.iov_len = sizeof(regs),
> +	};
> +
> +	unsigned long orig_a0;
> +	struct iovec a0_iov = {
> +		.iov_base = &orig_a0,
> +		.iov_len = sizeof(orig_a0),
> +	};
> +
> +	pid = fork();
> +	if (pid == 0) {
> +		/* Mark oneself being traced */
> +		long val = ptrace(PTRACE_TRACEME, 0, 0, 0);
> +		if (val)
> +			perr_and_exit("failed to request for tracer to trace me: %ld\n", val);
> +
> +		kill(getpid(), SIGSTOP);
> +
> +		/* Perform exit syscall that will be intercepted */
> +		exit(A0_OLD);
> +	}
> +
> +	if (pid < 0)
> +		exit(1);
> +
> +	if (waitpid(pid, &status, 0) != pid)
> +		perr_and_exit("failed to wait for the tracee %d\n", pid);
> +
> +	/* Stop at the entry point of the syscall */
> +	resume_and_wait_tracee(pid, PTRACE_SYSCALL);
> +
> +	/* Check tracee regs before the syscall */
> +	if (ptrace(PTRACE_GETREGSET, pid, NT_PRSTATUS, &iov))
> +		perr_and_exit("failed to get tracee registers\n");
> +	if (ptrace(PTRACE_GETREGSET, pid, NT_RISCV_ORIG_A0, &a0_iov))
> +		perr_and_exit("failed to get tracee registers\n");
> +	if (orig_a0 != A0_OLD)
> +		perr_and_exit("unexpected orig_a0: 0x%lx\n", orig_a0);
> +
> +	/* Modify a0/orig_a0 for the syscall */
> +	switch (opt) {
> +	case A0_MODIFY:
> +		regs.a0 = A0_NEW;
> +		break;

Did you mean applying the modified user_regs_struct using PTRACE_SETREGSET?
If yes, then there should be an appropriate PTRACE_SETREGSET NT_PRSTATUS call.
If no, then regs is ignored, so why would you change it in the first place?

> +	case ORIG_A0_MODIFY:
> +		orig_a0 = A0_NEW;
> +		break;
> +	}
> +
> +	if (ptrace(PTRACE_SETREGSET, pid, NT_RISCV_ORIG_A0, &a0_iov))
> +		perr_and_exit("failed to set tracee registers\n");
> +
> +	/* Resume the tracee */
> +	ptrace(PTRACE_CONT, pid, 0, 0);
> +	if (waitpid(pid, &status, 0) != pid)
> +		perr_and_exit("failed to wait for the tracee\n");
> +
> +	*result = WEXITSTATUS(status);
> +}

-- 
ldv

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