On Sat, Nov 16, 2019 at 11:08:06PM -0800, Andrii Nakryiko wrote:
> Add support for extern variables, provided to BPF program by libbpf. Currently
> the following extern variables are supported:
> - LINUX_KERNEL_VERSION; version of a kernel in which BPF program is
> executing, follows KERNEL_VERSION() macro convention;
> - CONFIG_xxx values; a set of values of actual kernel config. Tristate,
> boolean, and integer values are supported. Strings are not supported at
> the moment.
>
> All values are represented as 64-bit integers, with the follow value encoding:
> - for boolean values, y is 1, n or missing value is 0;
> - for tristate values, y is 1, m is 2, n or missing value is 0;
> - for integers, the values is 64-bit integer, sign-extended, if negative; if
> config value is missing, it's represented as 0, which makes explicit 0 and
> missing config value indistinguishable. If this will turn out to be
> a problem in practice, we'll need to deal with it somehow.
I read that statement as there is no extensibility for such api.
> Generally speaking, libbpf is not aware of which CONFIG_XXX values is of which
> expected type (bool, tristate, int), so it doesn't enforce any specific set of
> values and just parses n/y/m as 0/1/2, respectively. CONFIG_XXX values not
> found in config file are set to 0.
This is not pretty either.
> +
> + switch (*value) {
> + case 'n':
> + *ext_val = 0;
> + break;
> + case 'y':
> + *ext_val = 1;
> + break;
> + case 'm':
> + *ext_val = 2;
> + break;
> + case '"':
> + pr_warn("extern '%s': strings are not supported\n",
> + ext->name);
> + err = -EINVAL;
> + goto out;
> + default:
> + errno = 0;
> + *ext_val = strtoull(value, &value_end, 10);
> + if (errno) {
> + err = -errno;
> + pr_warn("extern '%s': failed to parse value: %d\n",
> + ext->name, err);
> + goto out;
> + }
BPF has bpf_strtol() helper. I think it would be cleaner to pass whatever
.config has as bytes to the program and let program parse n/y/m, strings and
integers.
LINUX_KERNEL_VERSION is a special case and can stay as u64.
