Official docs:
Some headers from C library were deprecated in C++ and are no longer
welcome in C++ codebases. Some have no effect in C++. For more details
refer to the C++ 14 Standard [depr.c.headers] section. This check
replaces C standard library headers with their C++ alternatives and
removes redundant ones.
For unaligned offset pread() may return EINVAL even if the offset pass
EOF, although it should not, since otherwise there is no abiliity to
rely on read() == 0 is EOF (with pread() loop).
Here is a reproducer for the problem on 4.9.0-12-amd64:
$ head -c27 /dev/urandom > /tmp/pread.issue
$ xfs_io
xfs_io> open -d /tmp/pread.issue
xfs_io> pread 1000 4096
pread: Invalid argument
And this is how it should work:
xfs_io> pread 29 4096
read 0/4096 bytes at offset 29
Note, here I use interactive mode since we had old xfs_io that does not
allow to execute multiple commands at once, and to avoid EMFILE issue
Here is some history of a patches that affects this behaviour in the
linux kernel:
- the issue had been introduced in
torvalds/linux@9fe55eea7e v3.14
("Fix race when checking i_size on direct i/o read")
- an attempt to fix it had been made in
torvalds/linux@74cedf9b6c v4.4
("direct-io: Fix negative return from dio read beyond eof")
- but this wasn't enough, since alignment check was earlier, so
eventually fixed in
torvalds/linux@41b21af388 v5.10
("direct-io: defer alignment check until after the EOF check")
Someone may ask why CI does not shows the issue, since:
- it had 4.19 kernel when CI was in yandex
- now it has 5.4 when CI is in AWS
Since both of those kernels does not have the last patch.
But, this bug requires the following conditions to met:
- index_granularity_bytes=0
- min_merge_bytes_to_use_direct_io=1
Which was not covered by CI yet.
Since this may create pretty odd issues, since reading 0 bytes will
return 0, and some code may not be ready for this.
v0: add a check in ReadBuffer ctor
v2: Do not create empty ReadBuffer from BufferWithOwnMemory with empty size
v3:
- revert "Do not create empty ReadBuffer from BufferWithOwnMemory with empty size"
- Replace INVALID_SETTING_VALUE with LOGICAL_ERROR
- Move the check for empty buffer in ReadBuffer into reading because of MMapReadBufferFromFile
v4: replace with assert of internal_buffer.size()
v5: move assertion to implementations since there are exceptions for
nested readers, like LimitReadBuffer and similar.
