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From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Florian Weimer <fw@deneb.enyo.de>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org, oleg@redhat.com,
	x86@kernel.org, libffi-discuss@sourceware.org, luto@kernel.org,
	David.Laight@ACULAB.COM, mark.rutland@arm.com, mic@digikod.net,
	pavel@ucw.cz
Subject: Re: [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor
Date: Wed, 23 Sep 2020 18:51:42 -0500	[thread overview]
Message-ID: <6409d394-9dd2-ae84-88fc-03218515d57d@linux.microsoft.com> (raw)
In-Reply-To: <20200923195147.GA1358246@rani.riverdale.lan>



On 9/23/20 2:51 PM, Arvind Sankar wrote:
> On Wed, Sep 23, 2020 at 02:17:30PM -0500, Madhavan T. Venkataraman wrote:
>>
>>
>> On 9/23/20 4:11 AM, Arvind Sankar wrote:
>>> For libffi, I think the proposed standard trampoline won't actually
>>> work, because not all ABIs have two scratch registers available to use
>>> as code_reg and data_reg. Eg i386 fastcall only has one, and register
>>> has zero scratch registers. I believe 32-bit ARM only has one scratch
>>> register as well.
>>
>> The trampoline is invoked as a function call in the libffi case. Any
>> caller saved register can be used as code_reg, can it not? And the
>> scratch register is needed only to jump to the code. After that, it
>> can be reused for any other purpose.
>>
>> However, for ARM, you are quite correct. There is only one scratch
>> register. This means that I have to provide two types of trampolines:
>>
>> 	- If an architecture has enough scratch registers, use the currently
>> 	  defined trampoline.
>>
>> 	- If the architecture has only one scratch register, but has PC-relative
>> 	  data references, then embed the code address at the bottom of the
>> 	  trampoline and access it using PC-relative addressing.
>>
>> Thanks for pointing this out.
>>
>> Madhavan
> 
> libffi is trying to provide closures with non-standard ABIs as well: the
> actual user function is standard ABI, but the closure can be called with
> a different ABI. If the closure was created with FFI_REGISTER abi, there
> are no registers available for the trampoline to use: EAX, EDX and ECX
> contain the first three arguments of the function, and every other
> register is callee-save.
> 
> I provided a sample of the kind of trampoline that would be needed in
> this case -- it's position-independent and doesn't clobber any registers
> at all, and you get 255 trampolines per page. If I take another 16-byte
> slot out of the page for the end trampoline that does the actual work,
> I'm sure I could even come up with one that can just call a normal C
> function, only the return might need special handling depending on the
> return type.
> 
> And again, do you actually have any example of an architecture that
> cannot run position-independent code? PC-relative addressing is an
> implementation detail: the fact that it's available for x86_64 but not
> for i386 just makes position-independent code more cumbersome on i386,
> but it doesn't make it impossible. For the tiny trampolines here, it
> makes almost no difference.
> 

Hi Arvind,

I am preparing a response for all of your comments. I will send it out
tomorrow. Sorry for the delay.

Madhavan

  reply	other threads:[~2020-09-23 23:51 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <210d7cd762d5307c2aa1676705b392bd445f1baa>
2020-09-16 15:08 ` madvenka
2020-09-16 15:08   ` [PATCH v2 1/4] [RFC] fs/trampfd: Implement the trampoline file descriptor API madvenka
2020-09-16 15:08   ` [PATCH v2 2/4] [RFC] x86/trampfd: Provide support for the trampoline file descriptor madvenka
2020-09-16 15:08   ` [PATCH v2 3/4] [RFC] arm64/trampfd: " madvenka
2020-09-16 15:08   ` [PATCH v2 4/4] [RFC] arm/trampfd: " madvenka
2020-09-17  1:04   ` [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor Florian Weimer
2020-09-17 15:36     ` Madhavan T. Venkataraman
2020-09-17 15:57       ` Madhavan T. Venkataraman
2020-09-17 16:01         ` Florian Weimer
2020-09-23  1:46       ` Arvind Sankar
2020-09-23  9:11         ` Arvind Sankar
2020-09-23 19:17           ` Madhavan T. Venkataraman
2020-09-23 19:51             ` Arvind Sankar
2020-09-23 23:51               ` Madhavan T. Venkataraman [this message]
2020-09-24 20:23               ` Madhavan T. Venkataraman
2020-09-24 20:52                 ` Florian Weimer
2020-09-25 22:22                   ` Madhavan T. Venkataraman
2020-09-27 18:25                     ` Madhavan T. Venkataraman
2020-09-24 22:13                 ` Pavel Machek
2020-09-24 23:43                 ` Arvind Sankar
2020-09-25 22:44                   ` Madhavan T. Venkataraman
2020-09-26 15:55                     ` Arvind Sankar
2020-09-27 17:59                       ` Madhavan T. Venkataraman
2020-09-22 21:53 ` madvenka
2020-09-22 21:53   ` [PATCH v2 1/4] [RFC] fs/trampfd: Implement the trampoline file descriptor API madvenka
2020-09-22 21:53   ` [PATCH v2 2/4] [RFC] x86/trampfd: Provide support for the trampoline file descriptor madvenka
2020-09-22 21:53   ` [PATCH v2 3/4] [RFC] arm64/trampfd: " madvenka
2020-09-22 21:53   ` [PATCH v2 4/4] [RFC] arm/trampfd: " madvenka
2020-09-22 21:54   ` [PATCH v2 0/4] [RFC] Implement Trampoline File Descriptor Madhavan T. Venkataraman
2020-09-23  8:14   ` Pavel Machek
2020-09-23  9:14     ` Solar Designer
2020-09-23 14:11       ` Solar Designer
2020-09-23 15:18         ` Pavel Machek
2020-09-23 18:00           ` Solar Designer
2020-09-23 18:21             ` Solar Designer
2020-09-23 14:39       ` Florian Weimer
2020-09-23 18:09         ` Andy Lutomirski
2020-09-23 18:11         ` Solar Designer
2020-09-23 18:49           ` Arvind Sankar
2020-09-23 23:53         ` Madhavan T. Venkataraman
2020-09-23 19:41       ` Madhavan T. Venkataraman
2020-09-23 18:10     ` James Morris
2020-09-23 18:32     ` Madhavan T. Venkataraman
2020-09-23  8:42   ` Pavel Machek
2020-09-23 18:56     ` Madhavan T. Venkataraman
2020-09-23 20:51       ` Pavel Machek
2020-09-23 23:04         ` Madhavan T. Venkataraman
2020-09-24 16:44         ` Mickaël Salaün
2020-09-24 22:05           ` Pavel Machek
2020-09-25 10:12             ` Mickaël Salaün

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