LLVM ERROR: Cannot select

[brendangregg]

# ./vfsslower| head -50
LLVM ERROR: Cannot select: 0x29e97a0: ch,glue = BPFISD::CALL 0x1e16d00, 0x29c4f50, 0x29b9240, 0x2ac89b8, 0x29e9550, 0x1e16d00:1 [ORD=4] [ID=40]
  0x29c4f50: i64 = TargetExternalSymbol'memset' [ID=9]
  0x29b9240: i64 = Register %R1 [ID=6]
  0x2ac89b8: i64 = Register %R2 [ID=7]
  0x29e9550: i64 = Register %R3 [ID=8]
  0x1e16d00: ch,glue = CopyToReg 0x1e17078, 0x29e9550, 0x2ac8640, 0x1e17078:1 [ORD=4] [ID=39]
    0x29e9550: i64 = Register %R3 [ID=8]
    0x2ac8640: i64 = Constant<1216> [ID=3]
    0x1e17078: ch,glue = CopyToReg 0x29b95b8, 0x2ac89b8, 0x29baa58, 0x29b95b8:1 [ORD=4] [ID=38]
      0x2ac89b8: i64 = Register %R2 [ID=7]
      0x29baa58: i64 = Constant<0> [ID=4]
      0x29b95b8: ch,glue = CopyToReg 0x29c4988, 0x29b9240, 0x29bb020 [ORD=4] [ID=37]
        0x29b9240: i64 = Register %R1 [ID=6]
        0x29bb020: i64 = FrameIndex<5> [ID=2]
In function: trace_entry

Same program as #209, still trying to exclude non-storage-device I/O. Attaching this to vfs_read():

int trace_entry(struct pt_regs *ctx, struct file *file)
{
[...]
                struct super_block mnt_sb = {}, *mnt_sbp = file->f_path.mnt->mnt_sb;
                bpf_probe_read(&mnt_sb, sizeof(mnt_sb), &mnt_sbp);

... was hoping something out of the super_block could identify if this was a storage-backed FS or not.

[drzaeus77]

The size of super_block is likely way too big (1216 bytes) to be stored on the stack, causing the llvm codegen to give up. You will unfortunately probably have to bpf_probe_read step by step until #188 is fixed.

struct vfsmount *mnt = file->f_path.mnt;
struct super_block *mnt_sb = NULL;
bpf_probe_read(&mnt_sb, sizeof(mnt_sb), &mnt->mnt_sb);
u32 s_flags = 0;
bpf_probe_read(&s_flags, sizeof(s_flags), &mnt_sb->s_flags);

[brendangregg]

Thanks for the code and explanation; that actually helps me understand the role of bpf_probe_read() much better -- it's to put things on the BPF stack (and satisfy the "expected=fp" messages)... Although maybe I don't need to learn it in the long term, if #188 is fixed.

Is there a way for bpf_probe_read() to check sizeof and the stack and print a better error for this?

[yonghong-song]

The issue cannot be reproduced any more, maybe because issue #188 has been fixed.