devpts: return ESPIPE for positional I/O on PTYs

[ryux1]

Description

PTY master and replica file descriptions currently fall through to the default
PRead and PWrite implementations, which return EINVAL. Mark both endpoints as
denying positional reads and writes so VFS returns ESPIPE before dispatching the
operation.

This matches Linux for preadv, preadv2, pwritev, and pwritev2 with explicit
offsets while preserving preadv2 and pwritev2 with offset -1 as streaming I/O.
It also lets callers such as Zig file writers recognize PTYs as unseekable and
fall back to normal streaming writes.

Fixes #13737.

Tests

• //test/syscalls:preadv2_test_runsc_systrap_directfs
• //test/syscalls:preadv2_test_runsc_ptrace
• bazel-bin/test/syscalls/linux/preadv2_test --gtest_filter=Preadv2Test.PtyWithOffset (native Linux)
• //test/syscalls:pwritev2_test_runsc_systrap_directfs
• //test/syscalls:pwritev2_test_runsc_ptrace
• //test/syscalls:pwritev2_test_native (focused PTY regression)
• //pkg/sentry/fsimpl/devpts:devpts_test
• //pkg/sentry/fsimpl/devpts:devpts_nogo
• //pkg/sentry/fsimpl/devpts:devpts_test_nogo

Assisted-by: Codex

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[konstantin-s-bogom]

While you're at this, can you also fix the preadv/preadv2 case as well?

[ryux1]

While you’re at this, can you also fix the preadv/preadv2 case as well?

Done — I added DenyPRead for both the PTY master and replica, plus coverage that verifies preadv/preadv2 with explicit offsets return ESPIPE. The test also checks that preadv2 with offset -1 still performs streaming reads in both directions.

Validated with the systrap/directfs and ptrace syscall targets, the focused native regression, and the devpts test/nogo targets.