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

Dmitry V. Levin
2025-01-14T17:04:01Z

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()").
=======

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