* Missing alignment in SFrame spec
@ 2023-04-28 8:08 Florian Weimer
2023-05-02 0:24 ` Indu Bhagat
0 siblings, 1 reply; 4+ messages in thread
From: Florian Weimer @ 2023-04-28 8:08 UTC (permalink / raw)
To: Indu Bhagat; +Cc: binutils
The SFrame specification does not say anything explict about alignment.
It uses ATTRIBUTE_PACKED, which strongly suggests alignment 1, but
looking at the structures themselves, I think what you actually want is
4-byte alignment.
Thanks,
Florian
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Missing alignment in SFrame spec
2023-04-28 8:08 Missing alignment in SFrame spec Florian Weimer
@ 2023-05-02 0:24 ` Indu Bhagat
2023-05-02 6:03 ` Florian Weimer
0 siblings, 1 reply; 4+ messages in thread
From: Indu Bhagat @ 2023-05-02 0:24 UTC (permalink / raw)
To: Florian Weimer; +Cc: binutils
On 4/28/23 01:08, Florian Weimer wrote:
> The SFrame specification does not say anything explict about alignment.
> It uses ATTRIBUTE_PACKED, which strongly suggests alignment 1, but
> looking at the structures themselves, I think what you actually want is
> 4-byte alignment.
>
> Thanks,
> Florian
>
Hi Florian,
SFrame stack trace format is an unaligned on-disk format. IIUC, you
suggest 4 byte alignment for all structs in SFrame format?
Yes, current SFrame FDE and SFrame FRE are 1-byte aligned.
I had reasoned about unaligned accesses previously when a bug showed up.
Memory READ side:
------------------
So far, we ran into some unaligned memory access issues earlier
https://sourceware.org/bugzilla/show_bug.cgi?id=29856, but were fixable
in libsframe, because the latter provides SFrame FREs via an in-memory
abstraction sframe_frame_row_entry defined in include/sframe-api.h. The
members of sframe_frame_row_entry are naturally aligned.
PR 29856 was about issues on the the memory READ side of SFrame FREs
(Frame Row Entry). Now, I had also expected to see unaligned access when
READ access to SFrame FDEs
(sframe_func_desc_entry.sfde_func_start_address) was done by GNU
ld/objdump. But when fixing the above bug, I did not see them. But I
have it noted down to get to this sometime soon.
Memory WRITE side:
------------------
As for the accesses to SFrame header and its members, these should all
be aligned memory accesses - both READ and WRITE. This is because SFrame
header is the first entity in the section. Its members have natural
alignment.
On the WRITE side, GAS does a byte by byte write IIRC (GAS does not use
libsframe APIs to write SFrame section). And as for GNU ld WRITE side, I
checked (mentally) that unaligned accesses are either taken care of in
libsframe or not present due to usage of memcpy.
So bottomline: I _think_ we are okay on the GNU as/ld READ and WRITE
side as far as unaligned memory accesses are concerned. *EXCEPT*, SFrame
Function Descriptor Entries (FDEs): there is an open item around access
to sframe_func_desc_entry.sfde_func_start_address (and following members
should exhibit the same problem), which I will check soon. I will double
check for both READ and WRITE access to all SFrame FDE members when I do
get to this.
Thank you for bringing it up. Does the above make sense ?
Indu
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Missing alignment in SFrame spec
2023-05-02 0:24 ` Indu Bhagat
@ 2023-05-02 6:03 ` Florian Weimer
2023-05-03 17:37 ` Indu Bhagat
0 siblings, 1 reply; 4+ messages in thread
From: Florian Weimer @ 2023-05-02 6:03 UTC (permalink / raw)
To: Indu Bhagat; +Cc: binutils
* Indu Bhagat:
> On 4/28/23 01:08, Florian Weimer wrote:
>> The SFrame specification does not say anything explict about alignment.
>> It uses ATTRIBUTE_PACKED, which strongly suggests alignment 1, but
>> looking at the structures themselves, I think what you actually want is
>> 4-byte alignment.
>> Thanks,
>> Florian
>>
>
> Hi Florian,
>
> SFrame stack trace format is an unaligned on-disk format. IIUC, you
> suggest 4 byte alignment for all structs in SFrame format?
>
> Yes, current SFrame FDE and SFrame FRE are 1-byte aligned.
>
> I had reasoned about unaligned accesses previously when a bug showed up.
Well, GAS seems to generate 8-byte-aligned .sframe sections (at least in
the 2.40-7.fc39 Fedora package). So all this is a bit inconsistent.
Thanks,
Florian
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Missing alignment in SFrame spec
2023-05-02 6:03 ` Florian Weimer
@ 2023-05-03 17:37 ` Indu Bhagat
0 siblings, 0 replies; 4+ messages in thread
From: Indu Bhagat @ 2023-05-03 17:37 UTC (permalink / raw)
To: Florian Weimer; +Cc: binutils
On 5/1/23 11:03 PM, Florian Weimer via Binutils wrote:
> * Indu Bhagat:
>
>> On 4/28/23 01:08, Florian Weimer wrote:
>>> The SFrame specification does not say anything explict about alignment.
>>> It uses ATTRIBUTE_PACKED, which strongly suggests alignment 1, but
>>> looking at the structures themselves, I think what you actually want is
>>> 4-byte alignment.
>>> Thanks,
>>> Florian
>>>
>>
>> Hi Florian,
>>
>> SFrame stack trace format is an unaligned on-disk format. IIUC, you
>> suggest 4 byte alignment for all structs in SFrame format?
>>
>> Yes, current SFrame FDE and SFrame FRE are 1-byte aligned.
>>
>> I had reasoned about unaligned accesses previously when a bug showed up.
>
> Well, GAS seems to generate 8-byte-aligned .sframe sections (at least in
> the 2.40-7.fc39 Fedora package). So all this is a bit inconsistent.
>
I will work on making the spec clear about alignment expectations for
the .sframe section and the various definitions in the format.
> Thanks,
> Florian
>
^ permalink raw reply [flat|nested] 4+ messages in thread
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2023-04-28 8:08 Missing alignment in SFrame spec Florian Weimer
2023-05-02 0:24 ` Indu Bhagat
2023-05-02 6:03 ` Florian Weimer
2023-05-03 17:37 ` Indu Bhagat
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