On Fri, Nov 13, 2020 at 11:36:08AM +0100, Björn Töpel wrote: > I was running the selftest/bpf on riscv, and had a closer look at one > of the failing cases: > > #14/p valid read map access into a read-only array 2 FAIL retval > 65507 != -29 (run 1/1) > > The test does a csum_partial() call via a BPF helper. riscv uses the > generic implementation. arm64 uses the generic csum_partial() and fail > in the same way [1]. arm (32-bit) has a arch specfic implementation, > and fail in another way (FAIL retval 131042 != -29) [2]. > > I mimicked the test case in a userland program, comparing the generic > csum_partial() to the x86 implementation [3], and the generic and x86 > implementation does yield a different result. > > x86 : -29 : 0xffffffe3 > generic : 65507 : 0x0000ffe3 > arm : 131042 : 0x0001ffe2 > > Who is correct? :-) It would be nice to get rid of this failed case... Don't expose unfolded csums to *anything* that might care about the specific bit pattern. All you are guaranteed is the value mod 0xffff. Full 32bit value is not just arch-specific - it can change from moving the area you are giving it by two bytes. Yes, really. It's *NOT* suitable for passig to userland. Or for sending over the wire. Or for storing in filesystem metadata (as reiserfs xattrs have done). __wsum is purely internal thing; BPF has no business sticking its fingers there, let alone exposing it as part of any kind of stable ABI.
