Re: [PATCH bpf-next 2/3] bpf: emit RECORD_MMAP events for bpf prog load/unload

[Song Liu <songliubraving@fb.com>]

> On Sep 19, 2018, at 3:39 PM, Alexei Starovoitov <ast@kernel.org> wrote:
> 
> use perf_event_mmap_bpf_prog() helper to notify user space
> about JITed bpf programs.
> Use RECORD_MMAP perf event to tell user space where JITed bpf program was loaded.
> Use empty program name as unload indication.
> 
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---
> kernel/bpf/core.c | 22 ++++++++++++++++++++--
> 1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 3f5bf1af0826..ddf11fdafd36 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -384,7 +384,7 @@ bpf_get_prog_addr_region(const struct bpf_prog *prog,
> 	*symbol_end   = addr + hdr->pages * PAGE_SIZE;
> }
> 
> -static void bpf_get_prog_name(const struct bpf_prog *prog, char *sym)
> +static char *bpf_get_prog_name(const struct bpf_prog *prog, char *sym)
> {
> 	const char *end = sym + KSYM_NAME_LEN;
> 
> @@ -402,9 +402,10 @@ static void bpf_get_prog_name(const struct bpf_prog *prog, char *sym)
> 	sym += snprintf(sym, KSYM_NAME_LEN, "bpf_prog_");
> 	sym  = bin2hex(sym, prog->tag, sizeof(prog->tag));
> 	if (prog->aux->name[0])
> -		snprintf(sym, (size_t)(end - sym), "_%s", prog->aux->name);
> +		sym += snprintf(sym, (size_t)(end - sym), "_%s", prog->aux->name);
> 	else
> 		*sym = 0;
> +	return sym;
> }
> 
> static __always_inline unsigned long
> @@ -480,23 +481,40 @@ static bool bpf_prog_kallsyms_verify_off(const struct bpf_prog *fp)
> 
> void bpf_prog_kallsyms_add(struct bpf_prog *fp)
> {
> +	unsigned long symbol_start, symbol_end;
> +	char buf[KSYM_NAME_LEN], *sym;
> +
> 	if (!bpf_prog_kallsyms_candidate(fp) ||
> 	    !capable(CAP_SYS_ADMIN))
> 		return;
> 
> +	bpf_get_prog_addr_region(fp, &symbol_start, &symbol_end);
> +	sym = bpf_get_prog_name(fp, buf);
> +	sym++; /* sym - buf is the length of the name including trailing 0 */
> +	while (!IS_ALIGNED(sym - buf, sizeof(u64)))
> +		*sym++ = 0;

nit: This logic feels a little weird to me. How about we wrap the extra logic
in a separate function:

size_t bpf_get_prog_name_u64_aligned(const struct bpf_prog fp, char *buf)

where the return value is the u64 aligned size. 

Other than this 

Acked-by: Song Liu <songliubraving@fb.com>

> 	spin_lock_bh(&bpf_lock);
> 	bpf_prog_ksym_node_add(fp->aux);
> 	spin_unlock_bh(&bpf_lock);
> +	perf_event_mmap_bpf_prog(symbol_start, symbol_end - symbol_start,
> +				 buf, sym - buf);
> }
> 
> void bpf_prog_kallsyms_del(struct bpf_prog *fp)
> {
> +	unsigned long symbol_start, symbol_end;
> +	/* mmap_record.filename cannot be NULL and has to be u64 aligned */
> +	char buf[sizeof(u64)] = {};
> +
> 	if (!bpf_prog_kallsyms_candidate(fp))
> 		return;
> 
> 	spin_lock_bh(&bpf_lock);
> 	bpf_prog_ksym_node_del(fp->aux);
> 	spin_unlock_bh(&bpf_lock);
> +	bpf_get_prog_addr_region(fp, &symbol_start, &symbol_end);
> +	perf_event_mmap_bpf_prog(symbol_start, symbol_end - symbol_start,
> +				 buf, sizeof(buf));
> }
> 
> static struct bpf_prog *bpf_prog_kallsyms_find(unsigned long addr)
> -- 
> 2.17.1
>