netdev - Re: [PATCHv2] btf_encoder: Match ftrace addresses within elf functions

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Date:   Wed, 17 Feb 2021 09:45:59 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Andrii Nakryiko <andrii@...nel.org>, dwarves@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Yonghong Song <yhs@...com>, Hao Luo <haoluo@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Sedat Dilek <sedat.dilek@...il.com>
Subject: Re: [PATCHv2] btf_encoder: Match ftrace addresses within elf
 functions

Em Sat, Feb 13, 2021 at 07:30:48PM -0700, Nathan Chancellor escreveu:
> On Sat, Feb 13, 2021 at 05:46:48PM +0100, Jiri Olsa wrote:
> > Currently when processing DWARF function, we check its entrypoint
> > against ftrace addresses, assuming that the ftrace address matches
> > with function's entrypoint.
> > 
> > This is not the case on some architectures as reported by Nathan
> > when building kernel on arm [1].
> > 
> > Fixing the check to take into account the whole function not
> > just the entrypoint.
> > 
> > Most of the is_ftrace_func code was contributed by Andrii.
> > 
> > [1] https://lore.kernel.org/bpf/20210209034416.GA1669105@ubuntu-m3-large-x86/
> > Acked-by: Andrii Nakryiko <andrii@...nel.org>
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> 
> I did several builds with CONFIG_DEBUG_INFO_BTF enabled (arm64, ppc64le,
> and x86_64) and saw no build errors. I did not do any runtime testing.
> 
> Tested-by: Nathan Chancellor <nathan@...nel.org>

Thanks, added to the cset,

- Arnaldo
 
> > ---
> > v2 changes:
> >   - update functions addr directly [Andrii]
> > 
> >  btf_encoder.c | 40 ++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 38 insertions(+), 2 deletions(-)
> > 
> > diff --git a/btf_encoder.c b/btf_encoder.c
> > index b124ec20a689..80e896961d4e 100644
> > --- a/btf_encoder.c
> > +++ b/btf_encoder.c
> > @@ -36,6 +36,7 @@ struct funcs_layout {
> >  struct elf_function {
> >  	const char	*name;
> >  	unsigned long	 addr;
> > +	unsigned long	 size;
> >  	unsigned long	 sh_addr;
> >  	bool		 generated;
> >  };
> > @@ -98,6 +99,7 @@ static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
> >  
> >  	functions[functions_cnt].name = name;
> >  	functions[functions_cnt].addr = elf_sym__value(sym);
> > +	functions[functions_cnt].size = elf_sym__size(sym);
> >  	functions[functions_cnt].sh_addr = sh.sh_addr;
> >  	functions[functions_cnt].generated = false;
> >  	functions_cnt++;
> > @@ -236,6 +238,39 @@ get_kmod_addrs(struct btf_elf *btfe, __u64 **paddrs, __u64 *pcount)
> >  	return 0;
> >  }
> >  
> > +static int is_ftrace_func(struct elf_function *func, __u64 *addrs, __u64 count)
> > +{
> > +	__u64 start = func->addr;
> > +	__u64 addr, end = func->addr + func->size;
> > +
> > +	/*
> > +	 * The invariant here is addr[r] that is the smallest address
> > +	 * that is >= than function start addr. Except the corner case
> > +	 * where there is no such r, but for that we have a final check
> > +	 * in the return.
> > +	 */
> > +	size_t l = 0, r = count - 1, m;
> > +
> > +	/* make sure we don't use invalid r */
> > +	if (count == 0)
> > +		return false;
> > +
> > +	while (l < r) {
> > +		m = l + (r - l) / 2;
> > +		addr = addrs[m];
> > +
> > +		if (addr >= start) {
> > +			/* we satisfy invariant, so tighten r */
> > +			r = m;
> > +		} else {
> > +			/* m is not good enough as l, maybe m + 1 will be */
> > +			l = m + 1;
> > +		}
> > +	}
> > +
> > +	return start <= addrs[r] && addrs[r] < end;
> > +}
> > +
> >  static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
> >  {
> >  	__u64 *addrs, count, i;
> > @@ -283,10 +318,11 @@ static int setup_functions(struct btf_elf *btfe, struct funcs_layout *fl)
> >  		 * functions[x]::addr is relative address within section
> >  		 * and needs to be relocated by adding sh_addr.
> >  		 */
> > -		__u64 addr = kmod ? func->addr + func->sh_addr : func->addr;
> > +		if (kmod)
> > +			func->addr += func->sh_addr;
> >  
> >  		/* Make sure function is within ftrace addresses. */
> > -		if (bsearch(&addr, addrs, count, sizeof(addrs[0]), addrs_cmp)) {
> > +		if (is_ftrace_func(func, addrs, count)) {
> >  			/*
> >  			 * We iterate over sorted array, so we can easily skip
> >  			 * not valid item and move following valid field into
> > -- 
> > 2.29.2
> > 

-- 

- Arnaldo