From: "H.J. Lu" <hjl.tools@gmail.com>
To: "Carlos O'Donell" <carlos@redhat.com>
Cc: Florian Weimer <fweimer@redhat.com>,
Szabolcs Nagy <nsz@port70.net>, Jan Beulich <JBeulich@suse.com>,
Rich Felker <dalias@libc.org>,
Binutils <binutils@sourceware.org>,
gnu-gabi@sourceware.org
Subject: Re: RFC: Add SHT_GNU_PHDRS
Date: Mon, 01 Jan 2018 00:00:00 -0000 [thread overview]
Message-ID: <CAMe9rOoCx4k4EMkB1kAqPuT6Y0mYN6RrOQ38FDf7fMmHgJDKJw@mail.gmail.com> (raw)
In-Reply-To: <0d529e75-beb2-3ba6-f4cc-e99d50880220@redhat.com>
[-- Attachment #1: Type: text/plain, Size: 5881 bytes --]
On Thu, Sep 27, 2018 at 8:18 AM, Carlos O'Donell <carlos@redhat.com> wrote:
> On 9/27/18 9:20 AM, H.J. Lu wrote:
>> On Thu, Sep 27, 2018 at 6:07 AM, Carlos O'Donell <carlos@redhat.com> wrote:
>>> On 9/27/18 8:57 AM, Florian Weimer wrote:
>>>> * H. J. Lu:
>>>>
>>>>> On Thu, Sep 27, 2018 at 5:42 AM, Florian Weimer <fweimer@redhat.com> wrote:
>>>>>> * H. J. Lu:
>>>>>>
>>>>>>> On Thu, Sep 27, 2018 at 3:35 AM, Szabolcs Nagy <nsz@port70.net> wrote:
>>>>>>>> an alloc .phdr section covering the program headers solves
>>>>>>>> this problem. if sections are not required for segments
>>>>>>>> then simply the linker should ensure that there is always
>>>>>>>> a load segment covering the program headers, possibly
>>>>>>>> without containing any sections, however elf says
>>>>>>>> "An object file segment contains one or more sections".
>>>>>>>>
>>>>>>>> i don't understand why a zero-size section is enough, what
>>>>>>>> if phdr > pagesize? will that get covered by the load
>>>>>>>> segment that is created for the zero-size section?
>>>>>>>
>>>>>>> Linker must keep this zero-size section in output and
>>>>>>> create a PT_LOAD segment to cover it even if it is
>>>>>>> the only SHF_ALLOC section in the PT_LOAD segment.
>>>>>>
>>>>>> Based on Szabolcs' comment, I don't think the section can be zero-sized.
>>>>>>
>>>>>
>>>>> Why can't we put a zero-size section in a PT_LOAD segment?
>>>>> Of course, we need to change linker to do it.
>>>>
>>>> I'm now under the impression that the bits that are PT_LOAD'ed all need
>>>> to be covered by (allocated) sections. A zero-sized section doesn't
>>>> cover anything, so it doesn't address this requirement of the ELF
>>
>> It depends on how we define it. I did experiment SHT_GNU_PHDRS
>> to cover the whole program header. But other tools don't expect a
>> section covering the program header.
>
> Which other tools? Specific examples please.
Please see the enclosed binary where SHT_GNU_PHDRS section
covers the whole program header:
[hjl@gnu-cfl-1 ld]$ ../binutils/readelf -lSW foo
There are 14 section headers, starting at offset 0x2f00:
Section Headers:
[Nr] Name Type Address Off Size
ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000
000000 00 0 0 0
[ 1] .gnu.phdrs GNU_PHDRS 0000000000400040 000040
0000e0 38 A 0 0 8
[ 2] .text PROGBITS 0000000000401000 001000
00014b 00 AX 0 0 16
[ 3] .rodata PROGBITS 0000000000402000 002000
000006 00 A 0 0 1
[ 4] .comment PROGBITS 0000000000000000 002006
00002c 01 MS 0 0 1
[ 5] .debug_aranges PROGBITS 0000000000000000 002040
000060 00 0 0 16
[ 6] .debug_info PROGBITS 0000000000000000 0020a0
000482 00 0 0 1
[ 7] .debug_abbrev PROGBITS 0000000000000000 002522
00016d 00 0 0 1
[ 8] .debug_line PROGBITS 0000000000000000 00268f
000248 00 0 0 1
[ 9] .debug_frame PROGBITS 0000000000000000 0028d8
000040 00 0 0 8
[10] .debug_str PROGBITS 0000000000000000 002918
000374 01 MS 0 0 1
[11] .symtab SYMTAB 0000000000000000 002c90
0001b0 18 12 12 8
[12] .strtab STRTAB 0000000000000000 002e40
000030 00 0 0 1
[13] .shstrtab STRTAB 0000000000000000 002e70
00008a 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)
Elf file type is EXEC (Executable file)
Entry point 0x401000
There are 4 program headers, starting at offset 64
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flg Align
LOAD 0x000000 0x0000000000400000 0x0000000000400000
0x000120 0x000120 R 0x1000
LOAD 0x001000 0x0000000000401000 0x0000000000401000
0x00014b 0x00014b R E 0x1000
LOAD 0x002000 0x0000000000402000 0x0000000000402000
0x000006 0x000006 R 0x1000
GNU_STACK 0x000000 0x0000000000000000 0x0000000000000000
0x000000 0x000000 RWE 0x10
Section to Segment mapping:
Segment Sections...
00 .gnu.phdrs
01 .text
02 .rodata
03
[hjl@gnu-cfl-1 ld]$ ./foo
PASS
[hjl@gnu-cfl-1 ld]$ ../binutils/objcopy ./foo bar
../binutils/objcopy: bar: section .gnu.phdrs lma 0x400040 adjusted to 0x401040
[hjl@gnu-cfl-1 ld]$
I can fix objcopy. Other tools may also need adjustment.
> The main problem we have to solve is:
>
> * Segfault when trying to access program headers which are expected to be
> mapped in by the leading pages of the PT_LOAD segment.
>
> We can't solve *all* the problems.
>
> The correct solution to the above is to improve the semantics that the
> toolchain relies upon to map the phdrs.
>
> Some questions which we might get asked is:
>
> * How does a running program know it's *safe* to look at it's own phdrs?
>
> * How many downstream tools are impacted? Do they really need to understand
> SHT_GNU_PHDRS?
>
>>>> specification.
>>>
>>> I agree. What we did in the past by relying on phdrs to be accidentally
>>> in the first PT_LOAD segment always irked me as bad design.
>>>
>>> If we need access to program header we need clear semantics for doing so,
>>> not hackish kludges to force the linker to get it onto a page that also
>>> happened to be mapped. This is just poor engineering on our part.
>>>
>>
>> My current dummy program property note section sounds much better
>> now :-).
>
> My apologies HJ, I did not intend this to sound like an attack on your
> original design, just that a new design like SHT_GNU_PHDRS could be
> created with reliable semantics.
None taken.
--
H.J.
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next prev parent reply other threads:[~2018-09-27 16:27 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-01 0:00 H.J. Lu
[not found] ` <CAORpzuOWtHeqBLEE+MMN4-TZyp6Z1r-MdmyNv7Zj-BhxMstr=g@mail.gmail.com>
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` Jan Beulich
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` Szabolcs Nagy
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` Carlos O'Donell
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Carlos O'Donell
2018-01-01 0:00 ` H.J. Lu [this message]
2018-01-01 0:00 ` Carlos O'Donell
2018-01-01 0:00 ` Cary Coutant
2018-01-01 0:00 ` H.J. Lu
2018-01-01 0:00 ` Rich Felker
2018-01-01 0:00 ` Michael Matz
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` Michael Matz
2018-01-01 0:00 ` Florian Weimer
2018-01-01 0:00 ` Cary Coutant
2018-01-01 0:00 ` Carlos O'Donell
2018-01-01 0:00 ` H.J. Lu
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