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Date: Wed, 29 Mar 2017 09:56:27 -0700 From: Andy Lutomirski <luto@...capital.net> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Oleg Nesterov <oleg@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>, Denys Vlasenko <dvlasenk@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, Jan Kratochvil <jan.kratochvil@...hat.com>, Pedro Alves <palves@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, "the arch/x86 maintainers" <x86@...nel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: syscall_get_error() && TS_ checks On Wed, Mar 29, 2017 at 9:45 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote: > On Wed, Mar 29, 2017 at 9:33 AM, Oleg Nesterov <oleg@...hat.com> wrote: >> >> Firstly, why do we need the IS_ERR_VALUE() check? This is only used by >> do_signal/handle_signal, we do not care if it returns non-zero as long >> as the value can't be confused with -ERESTART.* codes. > > There are system calls that can return "negative" values that aren't errors. > > Notably mmap() can return a valid pointer with the high bit set. > > So syscall_get_error() should return 0 for not just positive return > values, but for those kinds of negative non-error values. > >> And why do we need the TS_ checks? > > Those may be bogus. > >> So why we can't simply change putreg32() to always sign-extend regs->ax >> regs->orig_ax and just do >> >> static inline long syscall_get_error(struct task_struct *task, >> struct pt_regs *regs) >> { >> return regs-ax; >> } > > That would be *complete* garbage. Lots of system calls return positive > values that sure as hell aren't errors. Does this cause an observable problem? The only things that care are: a) 32-bit debugger pokes some value with the high bit and a 64-bit debugger reads it back. I seriously doubt we care. b) 32-bit debugger pokes some value with the high bit set and the user code switches to 64-bit mode and reads RAX. This case is so terminally broken anyway that we definitely don't care. c) 32-bit debugger pokes some value with the high bit set and syscall_get_error happens. Oleg's proposed change won't change what we do, but it will dramatically simplify the code.
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