Hi Albert,
Le 24/09/2014 09:34, Albert ARIBAUD a écrit :
Hi Georges,
On Sun, 21 Sep 2014 23:33:48 +0200, Georges Savoundararadj
<savou...@gmail.com> <mailto:savou...@gmail.com> wrote:
Before the commit 41623c91, the exception vector was in a .text
section which is allocatable. After this, the .vectors section was
introduced to contain the exception vector without specifying it as a
.text section but only as a executable section ("x").
This fix marks the section .vectors as allocatable as well ("ax") allowing
symbols to be relocated.
Could you explain (on the list, not in the commit message, although I
might ask you to do it there too later on) the exact effects of adding
the "a" flag, i.e., what symbol exactly was broken by commit 41623c91,
how it is broken, and how "a" changes this?
According to [1], the .text section has the attributes allocated and
executable:
"The attributes used are SHF_ALLOC and SHF_EXECINSTR".
Before commit 41623c91, the exception vectors were not defined in a
specific section.
For instance, see arch/arm/cpu/arm1176/start.S.
I think, if no specific section type is specified, the symbols will be,
by default, in a .text section.
It is what we observe thanks to arm-none-eabi-objdump -D
arch/arm/cpu/arm1176/start.o.
And, we notice that the addresses of the symbols of the exception
vectors are indeed there:
0003328c <__rel_dyn_end-0x4b00>:
3328c: 00008020 andeq r8, r0, r0, lsr #32
33290: 00000017 andeq r0, r0, r7, lsl r0
33294: 00008024 andeq r8, r0, r4, lsr #32
33298: 00000017 andeq r0, r0, r7, lsl r0
3329c: 00008028 andeq r8, r0, r8, lsr #32
332a0: 00000017 andeq r0, r0, r7, lsl r0
332a4: 0000802c andeq r8, r0, ip, lsr #32
8020 is the address of _undefined_instruction, 8024 of
_software_interrupt, etc., ...
In commit 41623c91, a specific section is created to contain these
symbols called .vectors
with the attribute executable (SHF_EXECINSTR or "x").
The symbols of the exception vectors are not referenced in .rel.dyn:
000332ac <__rel_dyn_end-0x4ac8>:
332ac: 00008538 andeq r8, r0, r8, lsr r5
332b0: 00000017 andeq r0, r0, r7, lsl r0
332b4: 0000853c andeq r8, r0, ip, lsr r5
332b8: 00000017 andeq r0, r0, r7, lsl r0
332bc: 00008540 andeq r8, r0, r0, asr #10
332c0: 00000017 andeq r0, r0, r7, lsl r0
332c4: 00008548 andeq r8, r0, r8, asr #10
332c8: 00000017 andeq r0, r0, r7, lsl r0
332cc: 0000854c andeq r8, r0, ip, asr #10
What has changed between commit 41623c91^ and 41623c91 is the attribute
of the section where the exception
vectors are.
If we add the lacking attribute (SHF_ALLOC or "a") for the definition of
the section .vectors, the problem is solved.
According to [1], a allocatable section is "a section [that] occupies
memory during process execution" which is
the case of .vectors.
To summarize, the issue is to set the section where the symbols
_undefined_instruction, _software_interrupt,
_prefetch_abort, _data_abort, _not_used, _irq and _fiq are as
allocatable because the exception vectors
resides in memory during execution.
If we do not mark it as allocatable, these symbols will no be referenced
for relocation.
[1] http://man7.org/linux/man-pages/man5/elf.5.html
On a deeper level, note that many ARM targets around do not have free
rein on where they can put vectors, e.g., they are limited to base
addresses of 0x00000000 or 0xFFFF0000. Thus, in many cases, vector are
relocated along with the rest of the image, but in fact, their base
address will not be their relocated address, and they will require
copying into whatever fixed location the target supports.
If you're going to fix general exception handling files (which I am
quite fine with), I would prefer that you solve the general problem,
i.e. either relocate vectors along with the code and fix VBAR on
plaforms that can use it, or relocate them to the fixed location wher
e the target expects them.
I didn't plan to to do that. But, I am ok to propose you a more general
patch.
Regards,
Georges
Signed-off-by: Georges Savoundararadj<savou...@gmail.com>
<mailto:savou...@gmail.com>
---
arch/arm/lib/vectors.S | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/arm/lib/vectors.S b/arch/arm/lib/vectors.S
index 0cb87ce..49238ed 100644
--- a/arch/arm/lib/vectors.S
+++ b/arch/arm/lib/vectors.S
@@ -33,7 +33,7 @@
*************************************************************************
*/
- .section ".vectors", "x"
+ .section ".vectors", "ax"
/*
*************************************************************************
Amicalement,
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