Re: [PATCH v3 3/6] syscall.h: introduce syscall_set_nr()

["Dmitry V. Levin" <ldv@strace.io>]

On Tue, Jan 28, 2025 at 04:13:52PM +0100, Christophe Leroy wrote:
> Le 28/01/2025 à 10:16, Dmitry V. Levin a écrit :
> > Similar to syscall_set_arguments() that complements
> > syscall_get_arguments(), introduce syscall_set_nr()
> > that complements syscall_get_nr().
> > 
> > syscall_set_nr() is going to be needed along with
> > syscall_set_arguments() on all HAVE_ARCH_TRACEHOOK
> > architectures to implement PTRACE_SET_SYSCALL_INFO API.
> > 
> > Signed-off-by: Dmitry V. Levin <ldv@strace.io>
> > Tested-by: Charlie Jenkins <charlie@rivosinc.com>
> > Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> >   arch/arc/include/asm/syscall.h        | 11 +++++++++++
> >   arch/arm/include/asm/syscall.h        | 24 ++++++++++++++++++++++++
> >   arch/arm64/include/asm/syscall.h      | 16 ++++++++++++++++
> >   arch/hexagon/include/asm/syscall.h    |  7 +++++++
> >   arch/loongarch/include/asm/syscall.h  |  7 +++++++
> >   arch/m68k/include/asm/syscall.h       |  7 +++++++
> >   arch/microblaze/include/asm/syscall.h |  7 +++++++
> >   arch/mips/include/asm/syscall.h       | 14 ++++++++++++++
> >   arch/nios2/include/asm/syscall.h      |  5 +++++
> >   arch/openrisc/include/asm/syscall.h   |  6 ++++++
> >   arch/parisc/include/asm/syscall.h     |  7 +++++++
> >   arch/powerpc/include/asm/syscall.h    | 10 ++++++++++
> >   arch/riscv/include/asm/syscall.h      |  7 +++++++
> >   arch/s390/include/asm/syscall.h       | 12 ++++++++++++
> >   arch/sh/include/asm/syscall_32.h      | 12 ++++++++++++
> >   arch/sparc/include/asm/syscall.h      | 12 ++++++++++++
> >   arch/um/include/asm/syscall-generic.h |  5 +++++
> >   arch/x86/include/asm/syscall.h        |  7 +++++++
> >   arch/xtensa/include/asm/syscall.h     |  7 +++++++
> >   include/asm-generic/syscall.h         | 14 ++++++++++++++
> >   20 files changed, 197 insertions(+)
> > 
> 
> > diff --git a/arch/arm64/include/asm/syscall.h b/arch/arm64/include/asm/syscall.h
> > index 76020b66286b..712daa90e643 100644
> > --- a/arch/arm64/include/asm/syscall.h
> > +++ b/arch/arm64/include/asm/syscall.h
> > @@ -61,6 +61,22 @@ static inline void syscall_set_return_value(struct task_struct *task,
> >   	regs->regs[0] = val;
> >   }
> >   
> > +static inline void syscall_set_nr(struct task_struct *task,
> > +				  struct pt_regs *regs,
> > +				  int nr)
> > +{
> > +	regs->syscallno = nr;
> > +	if (nr == -1) {
> > +		/*
> > +		 * When the syscall number is set to -1, the syscall will be
> > +		 * skipped.  In this case the syscall return value has to be
> > +		 * set explicitly, otherwise the first syscall argument is
> > +		 * returned as the syscall return value.
> > +		 */
> > +		syscall_set_return_value(task, regs, -ENOSYS, 0);
> > +	}
> > +}
> > +
> >   #define SYSCALL_MAX_ARGS 6
> >   
> >   static inline void syscall_get_arguments(struct task_struct *task,
> 
> > diff --git a/arch/powerpc/include/asm/syscall.h b/arch/powerpc/include/asm/syscall.h
> > index 521f279e6b33..7505dcfed247 100644
> > --- a/arch/powerpc/include/asm/syscall.h
> > +++ b/arch/powerpc/include/asm/syscall.h
> > @@ -39,6 +39,16 @@ static inline int syscall_get_nr(struct task_struct *task, struct pt_regs *regs)
> >   		return -1;
> >   }
> >   
> > +static inline void syscall_set_nr(struct task_struct *task, struct pt_regs *regs, int nr)
> > +{
> > +	/*
> > +	 * Unlike syscall_get_nr(), syscall_set_nr() can be called only when
> > +	 * the target task is stopped for tracing on entering syscall, so
> > +	 * there is no need to have the same check syscall_get_nr() has.
> > +	 */
> > +	regs->gpr[0] = nr;
> 
> Doesn't the same as for ARM64 apply here as well ?

I carefully checked all affected architectures and added that
syscall_set_return_value() call only where I think it's needed.

On powerpc it's not needed with the current implementation: their
do_seccomp() sets -ENOSYS before __secure_computing() invocation, and
their do_syscall_trace_enter() sets -ENOSYS in case of an invalid syscall
number.


-- 
ldv