|To:||"David S. Miller" <davem@xxxxxxxxxxxxx>|
|Subject:||Re: [PATCH/RFC] Reduce call chain length in netfilter|
|From:||Patrick McHardy <kaber@xxxxxxxxx>|
|Date:||Thu, 27 Jan 2005 18:50:50 +0100|
|Cc:||bdschuym@xxxxxxxxxx, netdev@xxxxxxxxxxx, netfilter-devel@xxxxxxxxxxxxxxxxxxx, snort2004@xxxxxxx, rusty@xxxxxxxxxxxxxxx, ak@xxxxxxx, bridge@xxxxxxxx, gandalf@xxxxxxxxxxxxxx, dwmw2@xxxxxxxxxxxxx, shemminger@xxxxxxxx|
|References:||<1131604877.20041218092730@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> <p73zn0ccaee.fsf@xxxxxxxxxxxxx> <1105117559.11753.34.camel@xxxxxxxxxxxxxxxxxxxxxxx> <20050107100017.454ddadc@xxxxxxxxxxxxxxxxx> <1105133241.3375.16.camel@xxxxxxxxxxxxxxxxxxxxx> <20050118135735.4b77d38d.davem@xxxxxxxxxxxxx> <1106433059.4486.11.camel@xxxxxxxxxxxxxxxxxxxxx> <1106436153.20995.42.camel@xxxxxxxxxxxxxx> <1106484019.3376.5.camel@xxxxxxxxxxxxxxxxxxxxx> <1106496509.1085.1.camel@xxxxxxxxxxxxxx> <20050125220558.6e824f8a.davem@xxxxxxxxxxxxx> <1106730510.4041.4.camel@xxxxxxxxxxxxxxxxxxxxx> <41F82C6D.7020006@xxxxxxxxx> <20050126231801.7bf90338.davem@xxxxxxxxxxxxx>|
|User-agent:||Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.7.5) Gecko/20050106 Debian/1.7.5-1|
David S. Miller wrote:
While reviewing I thought it may be an issue that the new macros potentially change skb. It really isn't an issue because NF_HOOK() calls pass ownership of the SKB over from the caller. Although technically, someone could go: skb_get(skb); err = NF_HOOK(... skb ...); ... do stuff with skb ... kfree_skb(skb); but that would cause other problems and I audited the entire tree and nobody attempts anything like this currently. 'skb' always dies at the NF_HOOK() call site.
Yes, it has always been illegal to use the skb after NF_HOOK.
Another huge downside to this change I was worried about was from a code generation point of view. Since we now take the address of "skb", gcc cannot generate tail-calls for the common case of: return NF_HOOK(...); when netfilter is enabled. Ho hum...
From what I can see it doesn't generate tail-calls currently: 34c: 45 31 c0 xor %r8d,%r8d 34f: 4c 89 e2 mov %r12,%rdx 352: be 01 00 00 00 mov $0x1,%esi 357: bf 02 00 00 00 mov $0x2,%edi 35c: c7 04 24 00 00 00 80 movl $0x80000000,(%rsp) 363: e8 00 00 00 00 callq 368 <ip_local_deliver+0x248>364: R_X86_64_PC32 nf_hook_slow+0xfffffffffffffffc
368: 48 83 c4 10 add $0x10,%rsp 36c: 5b pop %rbx 36d: 5d pop %rbp 36e: 41 5c pop %r12 370: c3 retq According to something I found on the internet, gcc only optimizes tail-calls if some conditions are met, in this case most importantly the space required for the arguments to the function called at the tail must not exceed the space required for the arguments of the function itself. nf_hook_slow takes 6 arguments, probably more than any caller. Regards Patrick
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