Re: [PATCH v2 1/7] powerpc: properly negate error in syscall_set_return_value()

From: Christophe Leroy
Date: Mon Jan 20 2025 - 09:20:19 EST




Le 14/01/2025 à 18:04, Dmitry V. Levin a écrit :
On Mon, Jan 13, 2025 at 06:34:44PM +0100, Christophe Leroy wrote:
Le 13/01/2025 à 18:10, Dmitry V. Levin a écrit :
Bring syscall_set_return_value() in sync with syscall_get_error(),
and let upcoming ptrace/set_syscall_info selftest pass on powerpc.

This reverts commit 1b1a3702a65c ("powerpc: Don't negate error in
syscall_set_return_value()").

There is a clear detailed explanation in that commit of why it needs to
be done.

If you think that commit is wrong you have to explain why with at least
the same level of details.

OK, please have a look whether this explanation is clear and detailed enough:

=======
powerpc: properly negate error in syscall_set_return_value()

When syscall_set_return_value() is used to set an error code, the caller
specifies it as a negative value in -ERRORCODE form.

In !trap_is_scv case the error code is traditionally stored as follows:
gpr[3] contains a positive ERRORCODE, and ccr has 0x10000000 flag set.
Here are a few examples to illustrate this convention. The first one
is from syscall_get_error():
/*
* If the system call failed,
* regs->gpr[3] contains a positive ERRORCODE.
*/
return (regs->ccr & 0x10000000UL) ? -regs->gpr[3] : 0;

The second example is from regs_return_value():
if (is_syscall_success(regs))
return regs->gpr[3];
else
return -regs->gpr[3];

The third example is from check_syscall_restart():
regs->result = -EINTR;
regs->gpr[3] = EINTR;
regs->ccr |= 0x10000000;

Compared with these examples, the failure of syscall_set_return_value()
to assign a positive ERRORCODE into regs->gpr[3] is clearly visible:
/*
* In the general case it's not obvious that we must deal with
* CCR here, as the syscall exit path will also do that for us.
* However there are some places, eg. the signal code, which
* check ccr to decide if the value in r3 is actually an error.
*/
if (error) {
regs->ccr |= 0x10000000L;
regs->gpr[3] = error;
} else {
regs->ccr &= ~0x10000000L;
regs->gpr[3] = val;
}

This fix brings syscall_set_return_value() in sync with syscall_get_error()
and lets upcoming ptrace/set_syscall_info selftest pass on powerpc.

Fixes: 1b1a3702a65c ("powerpc: Don't negate error in syscall_set_return_value()").
=======



I think there is still something going wrong.

do_seccomp() sets regs->gpr[3] = -ENOSYS; by default.

Then it calls __secure_computing() which returns what __seccomp_filter() returns.

In case of error, __seccomp_filter() calls syscall_set_return_value() with a negative value then returns -1

do_seccomp() is called by do_syscall_trace_enter() which returns -1 when do_seccomp() doesn't return 0.

do_syscall_trace_enter() is called by system_call_exception() and returns -1, so syscall_exception() returns regs->gpr[3]

In entry_32.S, transfer_to_syscall, syscall_exit_prepare() is then called with the return of syscall_exception() as first parameter, which leads to:

if (unlikely(r3 >= (unsigned long)-MAX_ERRNO) && is_not_scv) {
if (likely(!(ti_flags & (_TIF_NOERROR | _TIF_RESTOREALL)))) {
r3 = -r3;
regs->ccr |= 0x10000000; /* Set SO bit in CR */
}
}

By chance, because you have already changed the sign of gpr[3], the above test fails and nothing is done to r3, and because you have also already set regs->ccr it works.

But all this looks inconsistent with the fact that do_seccomp sets -ENOSYS as default value

Also, when do_seccomp() returns 0, do_syscall_trace_enter() check the syscall number and when it is wrong it goes to skip: which sets regs->gpr[3] = -ENOSYS;

So really I think it is not in line with your changes to set positive value in gpr[3].

Maybe your change is still correct but it needs to be handled completely in that case.

Christophe