From: Indu Bhagat <indu.bhagat@oracle.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: binutils@sourceware.org
Subject: Re: Missing alignment in SFrame spec
Date: Mon, 1 May 2023 17:24:49 -0700 [thread overview]
Message-ID: <2485551a-9608-e4ff-8d4c-bd7974c6c695@oracle.com> (raw)
In-Reply-To: <87ildgbf9g.fsf@oldenburg.str.redhat.com>
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
next prev parent reply other threads:[~2023-05-02 0:24 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-28 8:08 Florian Weimer
2023-05-02 0:24 ` Indu Bhagat [this message]
2023-05-02 6:03 ` Florian Weimer
2023-05-03 17:37 ` Indu Bhagat
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