Some of the future fields in struct pidfd_info can be optional. If the kernel has nothing to emit in that field, then it doesn't set the flag in the reply. This presents a problem: There is currently no way to know what mask flags the kernel supports since one can't always count on them being in the reply. Add a new PIDFD_INFO_SUPPORTED_MASK flag and field that the kernel can set in the reply. Userspace can use this to determine if the fields it requires from the kernel are supported. This also gives us a way to deprecate fields in the future, if that should become necessary.
Unified · Split
| @@ -293,6 +293,14 @@ static __u32 pidfs_coredump_mask(unsigned long mm_flags) · 292 unmodified lines | ||
| 293 | 293 | return 0; |
| 294 | 294 | } |
| 295 | 295 | |
| 296 | +/* This must be updated whenever a new flag is added */ | |
| 297 | +#define PIDFD_INFO_SUPPORTED (PIDFD_INFO_PID | \ | |
| 298 | + PIDFD_INFO_CREDS | \ | |
| 299 | + PIDFD_INFO_CGROUPID | \ | |
| 300 | + PIDFD_INFO_EXIT | \ | |
| 301 | + PIDFD_INFO_COREDUMP | \ | |
| 302 | + PIDFD_INFO_SUPPORTED_MASK) | |
| 303 | + | |
| 296 | 304 | static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg) |
| 297 | 305 | { |
| 298 | 306 | struct pidfd_info __user *uinfo = (struct pidfd_info __user *)arg; |
| @@ -306,7 +314,7 @@ static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg) · 7 unmodified lines | ||
| 306 | 314 | const struct cred *c; |
| 307 | 315 | __u64 mask; |
| 308 | 316 | |
| 309 | − BUILD_BUG_ON(sizeof(struct pidfd_info) != PIDFD_INFO_SIZE_VER1); | |
| 317 | + BUILD_BUG_ON(sizeof(struct pidfd_info) != PIDFD_INFO_SIZE_VER2); | |
| 310 | 318 | |
| 311 | 319 | if (!uinfo) |
| 312 | 320 | return -EINVAL; |
| @@ -412,6 +420,13 @@ static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg) · 99 unmodified lines | ||
| 412 | 420 | return -ESRCH; |
| 413 | 421 | |
| 414 | 422 | copy_out: |
| 423 | + if (mask & PIDFD_INFO_SUPPORTED_MASK) { | |
| 424 | + kinfo.mask |= PIDFD_INFO_SUPPORTED_MASK; | |
| 425 | + kinfo.supported_mask = PIDFD_INFO_SUPPORTED; | |
| 426 | + } | |
| 427 | + | |
| 428 | + /* Are there bits in the return mask not present in PIDFD_INFO_SUPPORTED? */ | |
| 429 | + WARN_ON_ONCE(~PIDFD_INFO_SUPPORTED & kinfo.mask); | |
| 415 | 430 | /* |
| 416 | 431 | * If userspace and the kernel have the same struct size it can just |
| 417 | 432 | * be copied. If userspace provides an older struct, only the bits that |
| @@ -26,9 +26,11 @@ · 25 unmodified lines | ||
| 26 | 26 | #define PIDFD_INFO_CGROUPID (1UL << 2) /* Always returned if available, even if not requested */ |
| 27 | 27 | #define PIDFD_INFO_EXIT (1UL << 3) /* Only returned if requested. */ |
| 28 | 28 | #define PIDFD_INFO_COREDUMP (1UL << 4) /* Only returned if requested. */ |
| 29 | +#define PIDFD_INFO_SUPPORTED_MASK (1UL << 5) /* Want/got supported mask flags */ | |
| 29 | 30 | |
| 30 | 31 | #define PIDFD_INFO_SIZE_VER0 64 /* sizeof first published struct */ |
| 31 | 32 | #define PIDFD_INFO_SIZE_VER1 72 /* sizeof second published struct */ |
| 33 | +#define PIDFD_INFO_SIZE_VER2 80 /* sizeof third published struct */ | |
| 32 | 34 | |
| 33 | 35 | /* |
| 34 | 36 | * Values for @coredump_mask in pidfd_info. |
| @@ -94,6 +96,7 @@ struct pidfd_info { · 59 unmodified lines | ||
| 94 | 96 | __s32 exit_code; |
| 95 | 97 | __u32 coredump_mask; |
| 96 | 98 | __u32 __spare1; |
| 99 | + __u64 supported_mask; /* Mask flags that this kernel supports */ | |
| 97 | 100 | }; |
| 98 | 101 | |
| 99 | 102 | #define PIDFS_IOCTL_MAGIC 0xFF |