Correct mempolicy syscalls and add setfs{uid,gid}

[jserv]

Dispatch table is generated from abi.h, so a wrong SYS_* number silently routes a handler to the wrong slot. SYS_get_mempolicy was defined as 239 and SYS_set_mempolicy as 238; the asm-generic numbers are 236 and 237. Guest calls to the real 236/237 fell through to unimplemented-syscall warning, while real 238/239 (migrate_pages, move_pages) silently invoked the misplaced mempolicy handlers: get_mempolicy on slot 239 wrote to whatever pointers the move_pages caller had loaded into x0/x1.

setfsuid (151) and setfsgid (152) were never wired. procps brackets its /proc walks with setfsuid(uid) / setfsuid(0), so ps and top warned twice on every refresh. Linux returns the previous fs{uid,gid} from these calls; elfuse does not track fsuid separately from euid, so the stub returns proc_get_{euid,egid}() and ignores the argument. procps observes a stable cred snapshot on both brackets, which is what its access-bracketing pattern needs. The glibc query idiom setfsuid(-1) also reports the current euid, so permission-validation callers (setfsuid(target); setfsuid(-1) != target) still detect a failed transition correctly.

The cred fields in proc-identity.c are _Atomic uint32_t, so unbracketed read in the stub is word-tear safe and matches the existing sc_geteuid pattern; CRED_BRACKETED is reserved for mutating syscalls that need to raise ATTN_BIT_CRED for sibling vCPUs.

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Summary by cubic

Corrected mempolicy syscall numbers and wired setfsuid/setfsgid. This fixes wrong syscall dispatch and removes ps/top warnings during /proc walks.

• Bug Fixes

  • Set mempolicy numbers to 236/237 in abi.h and dispatch; real 236/237 now handled, and 238/239 no longer hit the wrong handlers.
  • Updated tests to use the correct numbers.

• New Features

  • Added setfsuid/setfsgid stubs that return current euid/egid without changing state; supports the -1 query idiom.
  • Wired the syscalls in dispatch.tbl and added credential tests; procps /proc walks now see a stable snapshot and stop warning.

^{Written for commit faefa45. Summary will update on new commits.}