[Bug 1770480] Re: preadv2 test does not consider new flag from linux 4.16

[Thadeu Lima de Souza Cascardo]

Nominating bionic, as we are going to hit this issue there as soon as
some backport of linux appears on bionic.

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Title:
  preadv2 test does not consider new flag from linux 4.16

Status in glibc package in Ubuntu:
  New

Bug description:
  https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac
  /autopkgtest-cosmic-canonical-kernel-team-
  bootstrap/cosmic/arm64/g/glibc/20180510_173125_32961@/log.gz

  FAIL: misc/tst-preadvwritev2
  original exit status 1
  error: tst-preadvwritev2-common.c:45: preadv2 did not fail with an invalid flag
  error: 1 test failures
  ----------
  ----------
  FAIL: misc/tst-preadvwritev64v2
  original exit status 1
  error: tst-preadvwritev2-common.c:45: preadv2 did not fail with an invalid flag
  error: 1 test failures

  This is testing for the flag immediately after the last known flag.
  So, if it's 0x8, it's going to test for 0x10.

  The test snippet:

    int invalid_flag = RWF_SUPPORTED != 0 ? __builtin_clz (RWF_SUPPORTED) : 2;
    invalid_flag = 0x1 << ((sizeof (int) * CHAR_BIT) - invalid_flag);

    char buf[32];
    const struct iovec vec = { .iov_base = buf, .iov_len = sizeof (buf) };
    if (preadv2 (temp_fd, &vec, 1, 0, invalid_flag) != -1)
      FAIL_EXIT1 ("preadv2 did not fail with an invalid flag");


  However, linux 4.16 has introduced RWF_APPEND.

  commit e1fc742e14e01d84d9693c4aca4ab23da65811fb
  Author: Jürg Billeter <j at bitron.ch>
  Date:   Fri Sep 29 14:07:17 2017 +0200

      fs: add RWF_APPEND

  ---
  +/* per-IO O_APPEND */
  +#define RWF_APPEND     ((__force __kernel_rwf_t)0x00000010)
  +
   /* mask of flags supported by the kernel */
  -#define RWF_SUPPORTED  (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT)
  +#define RWF_SUPPORTED  (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
  +                        RWF_APPEND)

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