Re: [PATCH bpf-next 2/4] libbpf: Support 32-bit static data loads

[Joe Stringer <joe@wand.net.nz>]

On Thu, 14 Feb 2019 at 21:39, Y Song <ys114321@gmail.com> wrote:
>
> On Mon, Feb 11, 2019 at 4:48 PM Joe Stringer <joe@wand.net.nz> wrote:
> >
> > Support loads of static 32-bit data when BPF writers make use of
> > convenience macros for accessing static global data variables. A later
> > patch in this series will demonstrate its usage in a selftest.
> >
> > As of LLVM-7, this technique only works with 32-bit data, as LLVM will
> > complain if this technique is attempted with data of other sizes:
> >
> >     LLVM ERROR: Unsupported relocation: try to compile with -O2 or above,
> >     or check your static variable usage
>
> A little bit clarification from compiler side.
> The above compiler error is to prevent people use static variables since current
> kernel/libbpf does not handle this. The compiler only warns if .bss or
> .data section
> has more than one definitions. The first definition always has section offset 0
> and the compiler did not warn.

Ah, interesting. I observed that warning when I attempted to define
global variables of multiple sizes, and I thought also with sizes
other than 32-bit. This clarifies things a bit, thanks.

For the .bss my observation was that if you had a definition like:

static int a = 0;

Then this will be placed into .bss, hence why I looked into the
approach from this patch for patch 3 as well.

> The restriction is a little strange. To only work with 32-bit data is
> not a right
> statement. The following are some examples.
>
> The following static variable definitions will succeed:
> static int a; /* one in .bss */
> static long b = 2;  /* one in .data */
>
> The following definitions will fail as both in .bss.
> static int a;
> static int b;
>
> The following definitions will fail as both in .data:
> static char a = 2;
> static int b = 3;

Are there type restrictions or something? I've been defining multiple
static uint32_t and using them per the approach in this patch series
without hitting this compiler assertion.

> Using global variables can prevent compiler errors.
> maps are defined as globals and the compiler does not
> check whether a particular global variable is defining a map or not.
>
> If you just use static variable like below
> static int a = 2;
> without potential assignment to a, the compiler will replace variable
> a with 2 at compile time.
> An alternative is to define like below
> static volatile int a = 2;
> You can get a "load" for variable "a" in the bpf load even if there is
> no assignment to a.

I'll take a closer look at this too.

> Maybe now is the time to remove the compiler assertions as
> libbpf/kernel starts to
> handle static variables?

I don't understand why those assertions exists in this form. It
already allows code which will not load with libbpf (ie generate any
.data/.bss), does it help prevent unexpected situations for
developers?