[DTrace-devel] [PATCH] Improve error checking and reporting for BPF map creation

[Eugene Loh]

Reviewed-by: Eugene Loh <eugene.loh at oracle.com>
I guess all I'd add is that if the map create fails, it might be nice to 
report not just the map name but also ks, vs, and sz. That's less a 
fault of this patch as a fault of the existing code, but this patch 
might be a natural place to add such an enhancement.

On 8/9/22 14:43, Kris Van Hees via DTrace-devel wrote:
> Eugene Loh found that when BPF creation fails due to too big a map,
> E2BIG is returned.  This got reported as "Argument list too long"
> (using strerror()).  That is not very helpful.  It will now report
> "Too big".
>
> In addition, the key and value sizes are size_t entities in the main
> caller (dt_bpf_gmap_create()), while create_map() accepts ints for
> these values.  And the maximum number of map entries is an int in
> dt_bpf_gmap_create() but is at times calculated based on size_t values.
> And the bpf(BPF_MAP_CREATE) syscall expects uint32_t values for all
> three.  That is now cleaned up.
>
> The debug output in create_gmap() also got a small modification to no
> longer report sizes in the success report since it is already reported
> at the beginning of the function.
>
> Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
> ---
>   libdtrace/dt_bpf.c | 31 ++++++++++++++++++++-----------
>   1 file changed, 20 insertions(+), 11 deletions(-)
>
> diff --git a/libdtrace/dt_bpf.c b/libdtrace/dt_bpf.c
> index 5ff348ca..0c97ca7b 100644
> --- a/libdtrace/dt_bpf.c
> +++ b/libdtrace/dt_bpf.c
> @@ -100,8 +100,8 @@ int dt_bpf_prog_load(enum bpf_prog_type prog_type, const dtrace_difo_t *dp,
>    * Create a named BPF map.
>    */
>   int dt_bpf_map_create(enum bpf_map_type map_type, const char *name,
> -		      int key_size, int value_size, int max_entries,
> -		      uint32_t map_flags)
> +		      uint32_t key_size, uint32_t value_size,
> +		      uint32_t max_entries, uint32_t map_flags)
>   {
>   	union bpf_attr	attr;
>   
> @@ -242,26 +242,35 @@ dt_bpf_init_helpers(dtrace_hdl_t *dtp)
>   
>   static int
>   create_gmap(dtrace_hdl_t *dtp, const char *name, enum bpf_map_type type,
> -	    int ksz, int vsz, int size)
> +	    size_t ksz, size_t vsz, size_t size)
>   {
> -	int		fd;
> +	int		fd, err;
>   	dt_ident_t	*idp;
>   
> -	dt_dprintf("Creating BPF map '%s' (ksz %u, vsz %u, sz %d)\n",
> +	dt_dprintf("Creating BPF map '%s' (ksz %lu, vsz %lu, sz %lu)\n",
>   		   name, ksz, vsz, size);
> -	fd = dt_bpf_map_create(type, name, ksz, vsz, size, 0);
> +
> +	if (ksz > UINT_MAX || vsz > UINT_MAX || size > UINT_MAX) {
> +		fd = -1;
> +		err = E2BIG;
> +	} else {
> +		fd = dt_bpf_map_create(type, name, ksz, vsz, size, 0);
> +		err = errno;
> +	}
> +
>   	if (fd < 0) {
>   		char msg[64];
>   
>   		snprintf(msg, sizeof(msg),
>   			 "failed to create BPF map '%s'", name);
> -		if (errno == EPERM)
> +		if (err == E2BIG)
> +			return dt_bpf_error(dtp, "%s: Too big\n", msg);
> +		if (err == EPERM)
>   			return dt_bpf_lockmem_error(dtp, msg);
> -		return dt_bpf_error(dtp, "%s: %s\n", msg, strerror(errno));
> +		return dt_bpf_error(dtp, "%s: %s\n", msg, strerror(err));
>   	}
>   
> -	dt_dprintf("BPF map '%s' is FD %d (ksz %u, vsz %u, sz %d)\n",
> -		   name, fd, ksz, vsz, size);
> +	dt_dprintf("BPF map '%s' is FD %d\n", name, fd);
>   
>   	/*
>   	 * Assign the fd as id for the BPF map identifier.
> @@ -354,7 +363,7 @@ populate_probes_map(dtrace_hdl_t *dtp, int fd)
>   int
>   dt_bpf_gmap_create(dtrace_hdl_t *dtp)
>   {
> -	int		sz;
> +	size_t		sz;
>   	int		dvarc = 0;
>   	int		ci_mapfd, st_mapfd, pr_mapfd;
>   	uint64_t	key = 0;