Hi Matthew:
Firstly, thank you for your information.
form your answer, we understand the design intention of group, we Iook at
compilation proces and may be find the probelm of compiler, we the assume the
following scenario:
The client A and client B are in the same group, they all have the same
global variable like "global_buf", when use objcopy with
"--keep-global-symbol" param, it will change the "global_buf" symbol form
"gloabl" to "local", this change make the same name global variable of
different client in same space can work well, they can access their own global
variable as local variable.
But In out test, we find if the "global_buf" not be initialized, in the
group, there is only one
"global_buf", if client A change the value of "global_buf", client B also can
see the change, this is may be wrong, we find the gcc objcopy with
"--keep-global-symbol" can only affect ".data" and ".bss" section, but
"global_buf" which not be initialized, will place in ".common" section,
"--keep-global-symbol" will not change the symbol of ".common" form global to
local, this may be solved by add "-fno-common" when linker in camkes project
related to "group" scenario
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发件人: [email protected] <[email protected]>
发送时间: 2021年12月28日 9:00:05
收件人: [email protected]
主题: Devel Digest, Vol 130, Issue 2
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Today's Topics:
1. Re: Some questions about group of camkes (adjust the format)
(Matthew Fernandez)
----------------------------------------------------------------------
Message: 1
Date: Mon, 27 Dec 2021 16:26:26 -0800
From: Matthew Fernandez <[email protected]>
Subject: [seL4] Re: Some questions about group of camkes (adjust the
format)
To: "yadong.li" <[email protected]>
Cc: "[email protected]" <[email protected]>, "xiaonan.wang"
<[email protected]>, "lei.mao" <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
> On Dec 23, 2021, at 08:50, yadong.li <[email protected]> wrote:
>
> Hi
>
> * We learn the group of camkes from the
> https://github.com/seL4/camkes-tool/blob/master/docs/index.md
> * group can colocate two component instances in a single address space, we
> want to use the share var by this way, like directcall transfer string
> virtual address, But we feel confused about the global symbols who have the
> same symbol name include function and variable in a single address space,
> Their behavior seems undefined 。
My information on how this works is out of date and I can’t answer all your
questions, but I can tell you how this feature was originally implemented.
Single address space components were originally implemented using
post-compilation symbol mangling. As you’ve noticed, naive linking of two
component instances together presents two problems:
1. Unrelated homonyms between the instances now conflict. A symbol called `foo`
in component instance A and a symbol called `foo` in component instance B now
refer to the same thing when linked together.
2. References to glue code have the opposite problem: their names may differ in
component instance A and component instance B, but need to refer to the same
functions when the two are linked together.
Both problems were solved with the same mechanism: GNU objcopy. The invocations
to objcopy were template-generated so they could take advantage of knowledge
about the system begin compiled. A generated objcopy invocation would do the
following:
1. Adjust all non-generated symbols to have internal visibility. Component
instance A and component instance B have already been (partially) linked, so at
this point the only unresolved symbols that need to remain externally visible
are those related to the connection(s) between them. This solves problem 1
above.
2. Name-mangle all remaining symbols to something prefixed with the relevant
connection instance’s name. IIRC the name mangling scheme was something like
‘<connection instance name><space><original symbol name>’. This guaranteed
uniqueness because <space> is not a character that existed in C or assembly
symbols. Whether using a space in a symbol name is legal or not, I don’t know,
but all the binutils seemed fine with this.
3. Rename the second component in the name mangling above to a common name. I
don’t recall exactly how this name was chosen, but this solves problem 2 above.
This sounds pretty unorthodox and brittle, but it actually worked surprisingly
well. All combinations of single address space component systems seemed to Just
Work, with a few notable sharp edges:
* Anything involving MMIO was tricky. These symbols frequently needed to remain
externally visible and the two component instances would often have differing
names but the same addresses for them.
* GNU ld was more or less required. The multiple steps involving partial
linking was only supported by GNU ld and Gold at the time. LLVM’s lld may have
caught up in the interim years.
* The objcopy name mangling broke cross-section references used by GCC’s
implementation of Link-Time Optimization. As a result, any LTO compilation
degraded to LTO being disabled. This wouldn’t have been a big deal except that
one of the primary reasons to put two components in a single address space is
to enable cross-component inlining, usually facilitated by LTO. AFAICT this
(playing objcopy tricks and expecting LTO to still work) was simply not a
supported use case. We explored how to work around this and got some one-off
efforts working for benchmarking, but proper support would have involved
altering the way binutils work.
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