From: Fangrui Song <i@maskray.me>
To: Micha Nelissen <nelissen.micha@gmail.com>
Cc: musl@lists.openwall.com
Subject: Re: Re: size of executable
Date: Tue, 13 Aug 2019 11:45:43 +0000 [thread overview]
Message-ID: <20190813114251.jidhp3vgsvyidhnt@gmail.com> (raw)
In-Reply-To: <f99c8efc-9b8a-0963-f23d-32b343d49eaa@gmail.com>
On 2019-08-13, Micha Nelissen wrote:
>On 12-08-2019 20:23, Rich Felker wrote:
>>On Mon, Aug 12, 2019 at 11:16:39AM -0700, Khem Raj wrote:
>>>On Mon, Aug 12, 2019 at 10:19 AM Rich Felker <dalias@libc.org> wrote:
>>>>On Mon, Aug 12, 2019 at 05:55:28PM +0100, Jorge Almeida wrote:
>>>>>On Mon, Aug 12, 2019 at 5:48 PM Jorge Almeida <jjalmeida@gmail.com> wrote:
>>>>>>I get 16768 bytes (not stripped) and 12324 (stripped).
>>>>
>>>>This is a binutils regression from a dubious anti-ROP feature, -z
>>>>separate-code. Add -Wl,-z,noseparate-code and it will go away.
>>>
>>>is this still so with latest release as well.
>>
>>The breakage that caused separate-code to crash at runtime was fixed
>>between 2.31 and 2.32, but the size and performance regression
>>remains. With separate-code, a couple extra pages of memory and disk
>>are needed, with corresponding runtime cost to mmap them properly.
>>
>>All to avoid ROP gadgets, when every single dynamic-linked program has
>>a nice ROP gadget named "system" (among many others) in it...
>
>I'm curious. Jorge reports that the executable goes from 12k to 4k.
>That suggests two pages saved? But if I look at documentation for this
>separate-code option, then it says to allocate a separate code PT_LOAD
>segment. (PT_LOAD just means loadable?) That would suggest up to 4k
>more usage, not 8k right? One extra page necessary. Are by default
>rodata and code combined but with separate-code those are separated?
>Or something more happening?
binutils 2.31 includes a change "ld: Add --enable-separate-code". This is enabled by default for Linux x86.
You can compile a trivial program with -z separate-code and -z noseparate-code, run readelf -l and compare the results.
-z noseparate-code
LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x0007d8 0x0007d8 R E 0x1000
LOAD 0x000e18 0x0000000000001e18 0x0000000000001e18 0x000210 0x000218 RW 0x1000
-z separate-code
LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000530 0x000530 R 0x1000
LOAD 0x001000 0x0000000000001000 0x0000000000001000 0x0001cd 0x0001cd R E 0x1000
LOAD 0x002000 0x0000000000002000 0x0000000000002000 0x000148 0x000148 R 0x1000
LOAD 0x002e18 0x0000000000003e18 0x0000000000003e18 0x000210 0x000218 RW 0x1000
-z separate-code has two more PT_LOAD segments. What is bad is that the two PT_LOAD segments have aligned p_offset:
diff -u =(ld.bfd --verbose -z noseparate-code) =(ld.bfd --verbose -z separate-code)
+ . = ALIGN(CONSTANT (MAXPAGESIZE));
+ . = ALIGN(CONSTANT (MAXPAGESIZE));
+ /* Adjust the address for the rodata segment. We want to adjust up to
+ the same address within the page on the next page up. */
+ . = SEGMENT_START("rodata-segment", ALIGN(CONSTANT (MAXPAGESIZE)) + (. & (CONSTANT (MAXPAGESIZE) - 1)));
This disables an important size optimization (I have some description in https://reviews.llvm.org/D64906)
prev parent reply other threads:[~2019-08-13 11:45 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-12 16:48 Jorge Almeida
2019-08-12 16:55 ` Jorge Almeida
2019-08-12 17:18 ` Rich Felker
2019-08-12 17:59 ` Jorge Almeida
2019-08-12 18:16 ` Khem Raj
2019-08-12 18:23 ` Rich Felker
2019-08-13 10:15 ` Micha Nelissen
2019-08-13 11:45 ` Fangrui Song [this message]
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