On 05/02/2021 09.18, Thomas Huth wrote:
On 05/02/2021 08.08, Christian Borntraeger wrote:
On 05.02.21 07:12, Thomas Huth wrote:
On 04/02/2021 18.00, Christian Borntraeger wrote:
On 04.02.21 17:41, Thomas Huth wrote:
According to the "ELF-64 Object File Format" specification:
"The first word in the entry, namesz, identifies the length, in
bytes, of a name identifying the entry’s owner or originator. The
name field
contains a null-terminated string, with padding as necessary to
ensure 8-
byte alignment for the descriptor field. The length does not include
the
terminating null or the padding."
So we should not include the terminating NUL in the length field here.
Also there is a compiler warning with GCC 9.3 when compiling with
the -fsanitize=thread compiler flag:
In function 'strncpy',
inlined from 's390x_write_elf64_notes' at
../target/s390x/arch_dump.c:219:9:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: error:
'__builtin_strncpy' specified bound 8 equals destination size
[-Werror=stringop-truncation]
Since the name should always be NUL-terminated, let's use g_strlcpy() to
silence this warning. And while we're at it, also add an assert() to make
sure that the provided names always fit the size field (which is fine for
the current callers, the function is called once with "CORE" and once with
"LINUX" as a name).
Signed-off-by: Thomas Huth <th...@redhat.com>
---
v2: Use g_strlcpy instead of strncpy
With this patch I do get
WARNING: possibly corrupt Elf64_Nhdr: n_namesz: 0 n_descsz: 4 n_type: 88
when running crash on the elf file created by dump-guest-memory. Without
the
patch everything is fine.
Drat! Looking at the crash sources:
https://github.com/crash-utility/crash/blob/master/s390x.c#L378
... it seems like crash is rather rounding up to the next 4 bytes
boundary instead of the next 8 bytes boundary. Thus things go wrong now
when QEMU writes writes the "CORE" notes section. In the old code we were
using 4 + 1 as a lengths, so crash correctly rounded this up to 8. But
now with 4 as a length, this does not work right anymore :-(
Seems like I either misunderstood the "ELF-64 Object File Format"
specification, or this is a bug in the crash utility (it should either
add 1 to n_namesz for the trailing NUL or pad to 8 instead of 4)? Anyway,
it's maybe better to keep the "+ 1" in QEMU for now to avoid breaking
things, I guess?
I guess kdump and friends are also doing the +1 otherwise we would see the
error with those ELF dumps.
But yes, as long as crash does not work we must not apply this patch.
Looking at the other target/*/arch_dump.c files, it also seems like they
include the NUL in the namesz field for their 64-bit flavours ... I've got
the feeling that the "ELF-64 Object File Format" spec simply does not apply
here, or I must have missed something else.
After searching for a little bit longer, I guess this is the spec that
applies nowadays:
http://www.sco.com/developers/gabi/latest/ch5.pheader.html#note_section
Citing:
"The first namesz bytes in name contain a null-terminated character
representation of the entry's owner or originator."
So the NUL should be included in the namesz field, indeed.
Thomas
