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

[Nathan Chancellor <nathan@kernel.org>]

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@kernel.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.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@kernel.org>

> ---
> 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
>