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Date:   Tue, 27 Jun 2023 16:46:00 -0700
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Subject: Re: [PATCH v9 28/42] x86/shstk: Add user-mode shadow stack support
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To:     Mark Brown <broonie@kernel.org>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Arnd Bergmann <arnd@arndb.de>,
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        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H . J . Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
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        Randy Dunlap <rdunlap@infradead.org>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
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        david@redhat.com, debug@rivosinc.com, szabolcs.nagy@arm.com,
        torvalds@linux-foundation.org, Yu-cheng Yu <yu-cheng.yu@intel.com>,
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References: <20230613001108.3040476-1-rick.p.edgecombe@intel.com>
 <20230613001108.3040476-29-rick.p.edgecombe@intel.com>
 <76a87cdf-4d4f-4b3f-b01f-0540eab71ac7@sirena.org.uk>
From:   Dave Hansen <dave.hansen@intel.com>
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Precedence: bulk

On 6/27/23 10:20, Mark Brown wrote:
> On Mon, Jun 12, 2023 at 05:10:54PM -0700, Rick Edgecombe wrote:
> 
>> +static void unmap_shadow_stack(u64 base, u64 size)
>> +{
>> +	while (1) {
>> +		int r;
>> +
>> +		r = vm_munmap(base, size);
>> +
>> +		/*
>> +		 * vm_munmap() returns -EINTR when mmap_lock is held by
>> +		 * something else, and that lock should not be held for a
>> +		 * long time.  Retry it for the case.
>> +		 */
>> +		if (r == -EINTR) {
>> +			cond_resched();
>> +			continue;
>> +		}
> This looks generic, not even shadow stack specific - was there any
> discussion of making it a vm_munmap_retry() (that's not a great name...)
> or similar?  I didn't see any in old versions of the thread but I
> might've missed something.

Yeah, that looks odd.  Also odd is that none of the other users of
vm_munmap() bother to check the return value (except for one passthrough
in the nommu code).

I don't think the EINTR happens during contention, though.  It's really
there to be able break out in the face of SIGKILL.  I think that's why
nobody handles it: the task is dying anyway and nobody cares.

Rick, was this hunk here for a specific reason or were you just trying
to be diligent in handling errors?