Re: [Qemu-devel] [PULL] slirp: Fix issues with -mms-bitfields

[Stefan Weil <weil@mail.berlios.de>]

Am 15.08.2011 08:39, schrieb Jan Kiszka:
> The following changes since commit 
> 3b6ffe50300f13240e1b46420ad05da1116df410:
>
> hw/scsi-bus.c: Fix use of uninitialised variable (2011-08-14 19:34:25 
> +0000)
>
> are available in the git repository at:
> git://git.kiszka.org/qemu.git queues/slirp
>
> Jan Kiszka (1):
> slirp: Fix bit field types in IP header structs
>
> slirp/ip.h | 8 ++++----
> slirp/tcp.h | 4 ++--
> 2 files changed, 6 insertions(+), 6 deletions(-)
>
> ---
>
> slirp: Fix bit field types in IP header structs
>
> -mms-bitfields prevents that the bitfields in current IP header structs
> are packed into a single byte as it is required. Fix this by using
> uint8_t as backing type.
>
> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> ---
> slirp/ip.h | 8 ++++----
> slirp/tcp.h | 4 ++--
> 2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/slirp/ip.h b/slirp/ip.h
> index 48ea38e..72dbe9a 100644
> --- a/slirp/ip.h
> +++ b/slirp/ip.h
> @@ -74,10 +74,10 @@ typedef uint32_t n_long; /* long as received from 
> the net */
> */
> struct ip {
> #ifdef HOST_WORDS_BIGENDIAN
> - u_int ip_v:4, /* version */
> + uint8_t ip_v:4, /* version */
> ip_hl:4; /* header length */
> #else
> - u_int ip_hl:4, /* header length */
> + uint8_t ip_hl:4, /* header length */
> ip_v:4; /* version */
> #endif
> uint8_t ip_tos; /* type of service */
> @@ -140,10 +140,10 @@ struct ip_timestamp {
> uint8_t ipt_len; /* size of structure (variable) */
> uint8_t ipt_ptr; /* index of current entry */
> #ifdef HOST_WORDS_BIGENDIAN
> - u_int ipt_oflw:4, /* overflow counter */
> + uint8_t ipt_oflw:4, /* overflow counter */
> ipt_flg:4; /* flags, see below */
> #else
> - u_int ipt_flg:4, /* flags, see below */
> + uint8_t ipt_flg:4, /* flags, see below */
> ipt_oflw:4; /* overflow counter */
> #endif
> union ipt_timestamp {
> diff --git a/slirp/tcp.h b/slirp/tcp.h
> index 9d06836..b3817cb 100644
> --- a/slirp/tcp.h
> +++ b/slirp/tcp.h
> @@ -51,10 +51,10 @@ struct tcphdr {
> tcp_seq th_seq; /* sequence number */
> tcp_seq th_ack; /* acknowledgement number */
> #ifdef HOST_WORDS_BIGENDIAN
> - u_int th_off:4, /* data offset */
> + uint8_t th_off:4, /* data offset */
> th_x2:4; /* (unused) */
> #else
> - u_int th_x2:4, /* (unused) */
> + uint8_t th_x2:4, /* (unused) */
> th_off:4; /* data offset */
> #endif
> uint8_t th_flags;

Tested-by: Stefan Weil <weil@mail.berlios.de>

There remain some issues with other packed structs
which are obviously no longer packed with -mms-bitfields,
for example those from bt.h, but slirp looks good with
this patch.

I used pahole (thanks Blue Swirl for this hint) and codiff
to investigate and compare the structs with and without
-mms-bitfields:

* create qemu.exe (or any binary which you want)
* convert it to elf format using objcopy -Oelf32-i386
   (or the cross variant)
* apply pahole -a to the resulting elf file
   (without -a some structs are missing)

Cheers,
Stefan W.