Re: [PATCH v2 2/2] riscv: selftests: Add a ptrace test to verify syscall parameter modification

[Charlie Jenkins <charlie@rivosinc.com>]

On Tue, Dec 03, 2024 at 01:55:07PM +0100, Andrew Jones wrote:
> On Tue, Dec 03, 2024 at 05:30:05PM +0800, Celeste Liu wrote:
> > From: Charlie Jenkins <charlie@rivosinc.com>
> > 
> > This test checks that orig_a0 allows a syscall argument to be modified,
> > and that changing a0 does not change the syscall argument.
> > 
> > Cc: stable@vger.kernel.org
> > Co-developed-by: Quan Zhou <zhouquan@iscas.ac.cn>
> > Signed-off-by: Quan Zhou <zhouquan@iscas.ac.cn>
> > Co-developed-by: Celeste Liu <uwu@coelacanthus.name>
> > Signed-off-by: Celeste Liu <uwu@coelacanthus.name>
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> >  tools/testing/selftests/riscv/abi/.gitignore |   1 +
> >  tools/testing/selftests/riscv/abi/Makefile   |   5 +-
> >  tools/testing/selftests/riscv/abi/ptrace.c   | 134 +++++++++++++++++++++++++++
> >  3 files changed, 139 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tools/testing/selftests/riscv/abi/.gitignore b/tools/testing/selftests/riscv/abi/.gitignore
> > index b38358f91c4d2240ae64892871d9ca98bda1ae58..378c605919a3b3d58eec2701eb7af430cfe315d6 100644
> > --- a/tools/testing/selftests/riscv/abi/.gitignore
> > +++ b/tools/testing/selftests/riscv/abi/.gitignore
> > @@ -1 +1,2 @@
> >  pointer_masking
> > +ptrace
> > diff --git a/tools/testing/selftests/riscv/abi/Makefile b/tools/testing/selftests/riscv/abi/Makefile
> > index ed82ff9c664e7eb3f760cbab81fb957ff72579c5..3f74d059dfdcbce4d45d8ff618781ccea1419061 100644
> > --- a/tools/testing/selftests/riscv/abi/Makefile
> > +++ b/tools/testing/selftests/riscv/abi/Makefile
> > @@ -2,9 +2,12 @@
> >  
> >  CFLAGS += -I$(top_srcdir)/tools/include
> >  
> > -TEST_GEN_PROGS := pointer_masking
> > +TEST_GEN_PROGS := pointer_masking ptrace
> >  
> >  include ../../lib.mk
> >  
> >  $(OUTPUT)/pointer_masking: pointer_masking.c
> >  	$(CC) -static -o$@ $(CFLAGS) $(LDFLAGS) $^
> > +
> > +$(OUTPUT)/ptrace: ptrace.c
> > +	$(CC) -static -o$@ $(CFLAGS) $(LDFLAGS) $^
> > diff --git a/tools/testing/selftests/riscv/abi/ptrace.c b/tools/testing/selftests/riscv/abi/ptrace.c
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..d192764b428d1f1c442f3957c6fedeb01a48d556
> > --- /dev/null
> > +++ b/tools/testing/selftests/riscv/abi/ptrace.c
> > @@ -0,0 +1,134 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +#include <stdio.h>
> > +#include <stdlib.h>
> > +#include <string.h>
> > +#include <unistd.h>
> > +#include <fcntl.h>
> > +#include <signal.h>
> > +#include <errno.h>
> > +#include <sys/types.h>
> > +#include <sys/ptrace.h>
> > +#include <sys/stat.h>
> > +#include <sys/user.h>
> > +#include <sys/wait.h>
> > +#include <sys/uio.h>
> > +#include <linux/elf.h>
> > +#include <linux/unistd.h>
> > +#include <asm/ptrace.h>
> > +
> > +#include "../../kselftest_harness.h"
> > +
> > +#define ORIG_A0_MODIFY      0x01
> > +#define A0_MODIFY           0x02
> > +#define A0_OLD              0x03
> > +#define A0_NEW              0x04
> 
> Shouldn't A0_OLD and A0_NEW set more bits, since 3 and 4 aren't very
> unique (we could have those values by accident)? And should we include
> setting bits over 31 for 64-bit targets?
> 
> > +
> > +#define perr_and_exit(fmt, ...)						\
> > +	({								\
> > +		char buf[256];						\
> > +		snprintf(buf, sizeof(buf), "%s:%d:" fmt ": %m\n",	\
> > +			__func__, __LINE__, ##__VA_ARGS__);		\
> > +		perror(buf);						\
> > +		exit(-1);						\
> > +	})
> 
> Can we use e.g. ksft_exit_fail_perror() instead? I'd prefer we try to
> consolidate testing/selftests/riscv/* tests on a common format for
> errors and exit codes and we're already using other kselftest stuff.
> 
> > +
> > +static inline void resume_and_wait_tracee(pid_t pid, int flag)
> > +{
> > +	int status;
> > +
> > +	if (ptrace(flag, pid, 0, 0))
> > +		perr_and_exit("failed to resume the tracee %d\n", pid);
> > +
> > +	if (waitpid(pid, &status, 0) != pid)
> > +		perr_and_exit("failed to wait for the tracee %d\n", pid);
> > +}
> > +
> > +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);
> 
> This unexpected error condition deserves a message, so I'd use
> ksft_exit_fail_perror() here.
> 
> > +
> > +	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;
> > +	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);
> > +}
> > +
> > +TEST(ptrace_modify_a0)
> > +{
> > +	int result;
> > +
> > +	ptrace_test(A0_MODIFY, &result);
> > +
> > +	/* The modification of a0 cannot affect the first argument of the syscall */
> > +	EXPECT_EQ(A0_OLD, result);
> 
> What about checking that we actually set regs.a0 to A0_NEW? We'd need
> A0_NEW to be more unique than 4, though.
> 
> > +}
> > +
> > +TEST(ptrace_modify_orig_a0)
> > +{
> > +	int result;
> > +
> > +	ptrace_test(ORIG_A0_MODIFY, &result);
> > +
> > +	/* Only modify orig_a0 to change the first argument of the syscall */
> 
> If we run ptrace_modify_a0 first then we've already set regs.a0 to A0_NEW
> and can't check with this test that we don't set it to A0_NEW. We should
> probably have two different test values, one for regs.a0 and one for
> orig_a0 and ensure on both tests that we aren't writing both.
> 

Celeste, do you want to fix this up or are you waiting for me to?

- Charlie

> > +	EXPECT_EQ(A0_NEW, result);
> > +}
> > +
> > +TEST_HARNESS_MAIN
> > 
> > -- 
> > 2.47.0
> >
> 
> Thanks,
> drew