Implement posix_getdents

[RajeshRk18]

Closes #2034.

Adds sys_posix_getdents as specified by POSIX.1-2024 posix_getdents, alongside the existing sys_getdents64.

pub unsafe extern "C" fn sys_posix_getdents(fd: RawFd, buf: *mut PosixDent, nbyte: usize, flags: i32) -> isize

PosixDent matches struct posix_dent { d_ino, d_reclen, d_type, d_name[] } . flags must be 0 for now because POSIX defines no mandatory flags and DT_FORCE_TYPE is optional). Anything else returns EINVAL.

Implementation

• The entry-serialization logic that was duplicated across mem.rs and virtio_fs.rs is now a single DirentFormat::write_entry, parameterized over the two entry formats. ObjectInterface::getdents takes the requested DirentFormat.
• The default ObjectInterface::getdents now returns ENOTDIR instead of EINVAL, matching both POSIX and Linux getdents64(2) behavior for non-directory descriptors.

fixes surfaced by the new tests

• DirectoryReader::getdents was unimplemented!(), so sys_opendir + sys_getdents64 panicked the kernel. It is now implemented.
• MemDirectory::traverse_readdir never resolved the final path component.readdir("/tmp/gd") listed the contents of /tmp instead of /tmp/gd. Both went unnoticed because nothing exercised the sys_opendir path before.

Tests

New tests/getdents.rs integration test covers both syscalls through both the sys_opendir and O_DIRECTORY paths: entry layout/reclen for both formats, end-of-directory, partial reads with a one-entry buffer, rejected flags, EINVAL for a too-small buffer, and ENOTDIR for a file descriptor.

[github-actions]

Benchmark Results

Details

Benchmark	Current: fb91b72	Previous: 2e23902	Performance Ratio
startup_benchmark Build Time	94.66 s	80.34 s	1.18 ❗
startup_benchmark File Size	0.79 MB	0.80 MB	0.99 ❗
Startup Time - 1 core	0.74 s (±0.01 s)	0.75 s (±0.02 s)	0.99
Startup Time - 2 cores	0.75 s (±0.02 s)	0.74 s (±0.02 s)	1.02
Startup Time - 4 cores	0.75 s (±0.01 s)	0.74 s (±0.02 s)	1.01
multithreaded_benchmark Build Time	95.40 s	82.11 s	1.16 ❗
multithreaded_benchmark File Size	0.88 MB	0.86 MB	1.03 ❗
Multithreaded Pi Efficiency - 2 Threads	65.37 % (±6.44 %)	85.89 % (±6.61 %)	0.76 ❗
Multithreaded Pi Efficiency - 4 Threads	40.16 % (±2.89 %)	43.43 % (±2.56 %)	0.92
Multithreaded Pi Efficiency - 8 Threads	19.96 % (±1.93 %)	25.76 % (±1.53 %)	0.77 ❗
micro_benchmarks Build Time	214.79 s	80.40 s	2.67 ❗
micro_benchmarks File Size	0.89 MB	0.86 MB	1.03 ❗
Scheduling time - 1 thread	195.18 ticks (±36.34 ticks)	62.65 ticks (±4.06 ticks)	3.12 ❗
Scheduling time - 2 threads	105.96 ticks (±23.25 ticks)	34.08 ticks (±4.10 ticks)	3.11 ❗
Micro - Time for syscall (getpid)	11.54 ticks (±6.00 ticks)	3.45 ticks (±0.58 ticks)	3.35 ❗
Memcpy speed - (built_in) block size 4096	58067.78 MByte/s (±41174.50 MByte/s)	82448.38 MByte/s (±56997.13 MByte/s)	0.70
Memcpy speed - (built_in) block size 1048576	13884.88 MByte/s (±11637.67 MByte/s)	30585.98 MByte/s (±24707.84 MByte/s)	0.45
Memcpy speed - (built_in) block size 16777216	11478.28 MByte/s (±9800.74 MByte/s)	26340.06 MByte/s (±21720.96 MByte/s)	0.44
Memset speed - (built_in) block size 4096	58571.80 MByte/s (±41504.68 MByte/s)	82292.76 MByte/s (±56891.50 MByte/s)	0.71
Memset speed - (built_in) block size 1048576	14284.13 MByte/s (±11911.84 MByte/s)	31323.85 MByte/s (±25145.86 MByte/s)	0.46
Memset speed - (built_in) block size 16777216	11805.78 MByte/s (±10012.50 MByte/s)	27104.68 MByte/s (±22209.94 MByte/s)	0.44
Memcpy speed - (rust) block size 4096	51137.89 MByte/s (±37079.66 MByte/s)	74097.96 MByte/s (±51811.44 MByte/s)	0.69
Memcpy speed - (rust) block size 1048576	14212.71 MByte/s (±12020.82 MByte/s)	30361.60 MByte/s (±24602.37 MByte/s)	0.47
Memcpy speed - (rust) block size 16777216	10645.96 MByte/s (±8928.89 MByte/s)	27625.34 MByte/s (±22806.88 MByte/s)	0.39
Memset speed - (rust) block size 4096	51257.40 MByte/s (±37149.43 MByte/s)	74373.47 MByte/s (±51976.48 MByte/s)	0.69
Memset speed - (rust) block size 1048576	14528.18 MByte/s (±12201.79 MByte/s)	31110.89 MByte/s (±25033.24 MByte/s)	0.47
Memset speed - (rust) block size 16777216	11133.34 MByte/s (±9368.49 MByte/s)	28386.93 MByte/s (±23265.03 MByte/s)	0.39
alloc_benchmarks Build Time	224.58 s	74.76 s	3.00 ❗
alloc_benchmarks File Size	0.87 MB	0.87 MB	0.99 ❗
Allocations - Allocation success	91.38 %	91.31 %	1.00 ❗
Allocations - Deallocation success	100.00 %	100.00 %	1
Allocations - Pre-fail Allocations	61.60 %	61.44 %	1.00 ❗
Allocations - Average Allocation time	20975.80 Ticks (±1322.15 Ticks)	5860.58 Ticks (±98.43 Ticks)	3.58 ❗
Allocations - Average Allocation time (no fail)	22854.28 Ticks (±1610.76 Ticks)	6554.81 Ticks (±92.86 Ticks)	3.49 ❗
Allocations - Average Deallocation time	10123.07 Ticks (±1259.24 Ticks)	1805.01 Ticks (±250.35 Ticks)	5.61 ❗
mutex_benchmark Build Time	216.52 s	79.82 s	2.71 ❗
mutex_benchmark File Size	0.89 MB	0.86 MB	1.03 ❗
Mutex Stress Test Average Time per Iteration - 1 Threads	34.82 ns (±7.91 ns)	12.10 ns (±0.41 ns)	2.88 ❗
Mutex Stress Test Average Time per Iteration - 2 Threads	32.22 ns (±10.40 ns)	40.26 ns (±1.68 ns)	0.80

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