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Message-ID: <4022cb20af2759d0e71f72a1b4161b3e43181bca.camel@intel.com>
Date: Fri, 18 Aug 2023 13:07:14 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
"jarkko@...nel.org" <jarkko@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"Van Bulck, Jo" <jo.vanbulck@...kuleuven.be>
CC: "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 4/5] selftests/sgx: Ensure expected enclave data buffer
size and placement.
On Mon, 2023-08-07 at 11:41 +0200, Jo Van Bulck wrote:
> On 28.07.23 21:19, Jarkko Sakkinen wrote:
> > So, when exactly is it optimized away by the compiler? This is missing.
>
> The problem is that declaring encl_buf as static, implies that it will
> only be used in this file and the compiler is allowed to optimize away
> any entries that are never used within this compilation unit (e.g., when
> optimizing out the memcpy calls).
>
> In reality, the tests outside test_encl.elf rely on both the size and
> exact placement of encl_buf at the start of .data.
>
> For example, clang -Os generates the following (legal) code when
> encl_bug is declared as static:
>
> 0000000000001020 <do_encl_op_put_to_buf>:
> mov 0x8(%rdi),%al
> mov %al,0x1fd7(%rip) # 3000 <encl_buffer.0>
> mov 0x9(%rdi),%al
> mov %al,0x8fce(%rip) # a000 <encl_buffer.1.0>
> mov 0xa(%rdi),%al
> mov %al,0x8fd5(%rip) # a010 <encl_buffer.1.1>
> mov 0xb(%rdi),%al
> mov %al,0x8fce(%rip) # a012 <encl_buffer.1.2>
> mov 0xc(%rdi),%al
> mov %al,0x8fd3(%rip) # a020 <encl_buffer.1.3>
> mov 0xd(%rdi),%al
> mov %al,0x8fce(%rip) # a024 <encl_buffer.1.4>
> mov 0xe(%rdi),%al
> mov %al,0x8fd1(%rip) # a030 <encl_buffer.1.5>
> mov 0xf(%rdi),%al
> mov %al,0x8fca(%rip) # a032 <encl_buffer.1.6>
> ret
>
> Disassembly of section .data:
>
> 0000000000003000 <encl_buffer.0>:
> 3000: 01 00
> ...
> 0000000000004000 <encl_ssa_tcs1>:
>
> Thus, this proposed patch fixes both the size and location:
>
> 1. removing the static keyword from the encl_bug declaration ensures
> that the _entire_ buffer is preserved with expected size, as the
> compiler cannot anymore assume encl_buf is only used in this file.
Could we use "used" attribute?
https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html
used
This attribute, attached to a variable with static storage, means that
the variable must be emitted even if it appears that the variable is
not referenced.
When applied to a static data member of a C++ class template, the
attribute also means that the member is instantiated if the class
itself is instantiated.
>
> 2. adding _attribute__((section(".data.encl_buffer"))) ensures that we
> can control the expected location at the start of the .data section. I
> think this is optional, as encl_buf always seems to be placed at the
> start of .data in all my tests. But afaik this is not guaranteed as per
> the C standard and such constraints on exact placement should better be
> explicitly controlled in the linker script(?)
This looks sane.
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