bpf, s390: Mask the count of 32-bit shifts by register

RFC9669 defines the shifts as dst <<= (src & mask), dst >>= (src & mask)
and sign extending dst >>= (src & mask), where "shift operations use a mask
of 0x3F (63) for 64-bit operations and 0x1F (31) for 32-bit operations"
(Section 4.1), which the interpreter implements. On x86-64 and arm64 the
hardware does so by itself, so both JITs emit the shift alone and conform.

The 32-bit shifts by register of the s390x JIT do not conform to the spec.
They emit sll, srl and sra with the count in the address of the second
operand, of which the "rightmost six bits indicate the number of bit
positions to be shifted" (z/Architecture Principles of Operation,
SA22-7832-13), so a count of 32 to 63 clears the register, or fills it
with the sign for sra, instead of shifting by the count masked with 31.
The 64-bit sllg, srlg and srag take six bits as well, which is the mask
there, so only the 32-bit forms differ.

Put the count masked with 31 into %w1 and shift by that. A single risbg
does the copy and the mask: it inserts bits 59 to 63 of the source into
the same positions of %w1 and zeroes the rest, available since the
general-instructions-extension facility of z10, below the z196 the JIT
requires. One instruction more per shift by register, shifts by immediate
are not affected since the verifier rejects a count at or beyond the
width.

  # KERNEL_CMDLINE_EXTRA=nokaslr PLATFORM=s390x CROSS_COMPILE=s390x-linux-gnu- ./vmtest.sh -- \
    ./test_progs -t jit_insns/shift_counts,jit_insns/shifts,jit_insns/alu_edges,jit_insns/calls
  [...]
  #191/8   jit_insns/alu_edges:OK
  #191/9   jit_insns/shifts:OK
  #191/10  jit_insns/shift_counts:OK
  #191/21  jit_insns/calls:OK
  #191/22  jit_insns/helper_calls:OK
  #191     jit_insns:OK
  #192/3   jit_insns_blinded/alu_edges:OK
  #192/5   jit_insns_blinded/shifts:OK
  #192     jit_insns_blinded:OK
  Summary: 2/7 PASSED, 0 SKIPPED, 0/0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261016101822.632891-1-daniel@iogearbox.net

0 files changed, 0 insertions, 0 deletions