devpts: return ESPIPE for positional I/O on PTYs - #14111
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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 FUTURE_COPYBARA_INTEGRATE_REVIEW=#14097 from ryux1:fix/pty-positional-write-espipe 7308f5f PiperOrigin-RevId: 964247273
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devpts: return ESPIPE for positional I/O on PTYs
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
Assisted-by: Codex
FUTURE_COPYBARA_INTEGRATE_REVIEW=#14097 from ryux1:fix/pty-positional-write-espipe 7308f5f