Subject: Re: [PATCH net-next v2 2/3] net: skb: use auto-generation to convert skb drop reason to string

From: Jakub Kicinski <[email-masked]>
Date: Mon, 30 May 2022 13:13:11 -0700

On Mon, 30 May 2022 16:12:00 +0800 [email-masked] wrote:
> From: Menglong Dong <[email-masked]>
> 
> It is annoying to add new skb drop reasons to 'enum skb_drop_reason'
> and TRACE_SKB_DROP_REASON in trace/event/skb.h, and it's easy to forget
> to add the new reasons we added to TRACE_SKB_DROP_REASON.
> 
> TRACE_SKB_DROP_REASON is used to convert drop reason of type number
> to string. For now, the string we passed to user space is exactly the
> same as the name in 'enum skb_drop_reason' with a 'SKB_DROP_REASON_'
> prefix. Therefore, we can use 'auto-generation' to generate these
> drop reasons to string at build time.
> 
> The new header 'dropreason_str.h'

Not any more.

> will be generated, and the
> __DEFINE_SKB_DROP_REASON() in it can do the converting job. Meanwhile,
> we use a global array to store these string, which can be used both
> in drop_monitor and 'kfree_skb' tracepoint.

> diff --git a/include/net/dropreason.h b/include/net/dropreason.h
> index ecd18b7b1364..460de425297c 100644
> --- a/include/net/dropreason.h
> +++ b/include/net/dropreason.h
> @@ -3,6 +3,8 @@
>  #ifndef _LINUX_DROPREASON_H
>  #define _LINUX_DROPREASON_H
>  
> +#include <linux/kernel.h>

Why?

> +dropreason_str.c
> \ No newline at end of file

Heed the warning.

> diff --git a/net/core/Makefile b/net/core/Makefile
> index a8e4f737692b..3c7f99ff6d89 100644
> --- a/net/core/Makefile
> +++ b/net/core/Makefile
> @@ -4,7 +4,8 @@
>  #
>  
>  obj-y := sock.o request_sock.o skbuff.o datagram.o stream.o scm.o \
> -	 gen_stats.o gen_estimator.o net_namespace.o secure_seq.o flow_dissector.o
> +	 gen_stats.o gen_estimator.o net_namespace.o secure_seq.o \
> +	 flow_dissector.o dropreason_str.o
>  
>  obj-$(CONFIG_SYSCTL) += sysctl_net_core.o
>  
> @@ -39,3 +40,23 @@ obj-$(CONFIG_NET_SOCK_MSG) += skmsg.o
>  obj-$(CONFIG_BPF_SYSCALL) += sock_map.o
>  obj-$(CONFIG_BPF_SYSCALL) += bpf_sk_storage.o
>  obj-$(CONFIG_OF)	+= of_net.o
> +
> +clean-files := dropreason_str.c
> +
> +quiet_cmd_dropreason_str = GEN     $@
> +cmd_dropreason_str = awk -F ',' 'BEGIN{ print "\#include <net/dropreason.h>\n"; \
> +	print "const char * const drop_reasons[] = {" }\
> +	/^enum skb_drop/ { dr=1; }\
> +	/\}\;/ { dr=0; }\
> +	/^\tSKB_DROP_REASON_/ {\
> +		if (dr) {\
> +			sub(/\tSKB_DROP_REASON_/, "", $$1);\
> +			printf "\t[SKB_DROP_REASON_%s] = \"%s\",\n", $$1, $$1;\
> +		}\
> +	} \
> +	END{ print "};\nEXPORT_SYMBOL(drop_reasons);" }' $< > $@
> +
> +$(obj)/dropreason_str.c: $(srctree)/include/net/dropreason.h
> +	$(call cmd,dropreason_str)

I'm getting this:

  awk: cmd. line:1: warning: regexp escape sequence `\;' is not a known regexp operator