Re: [musl] Issue with fread() and unaligned readv()

[Rich Felker <dalias@libc.org>]

On Mon, Mar 15, 2021 at 05:39:43PM -0400, Dominic Chen wrote:
> Not sure this counts as a problem in musl or the application, but
> I've been debugging a return error of EINVAL from `fread(&buf, 8,
> 16, f)`, where `f = fopen("/proc/self/pagemap", "r")`. Internally,
> musl converts this into a call to `readv(f->fd, iov, 2)`, where `iov
> = {{iov_base = buf, iov_len = 127}, {iov_base = f->buf, iov_len =
> 1024}}`. However, it turns out that the kernel VFS read
> implementation inside `pagemap_read` checks that both the file
> position and count are divisible by PM_ENTRY_BYTES (8 on x86_64),
> otherwise it rejects the read with EINVAL. In comparison, glibc's
> `_IO_file_xsgetn` does appear to try to maintain read alignment,
> although I haven't looked at it in detail.

You can't use stdio to read or write special files/devices that depend
on the reads or writes happening in particular units, because the
relationship between stdio operations and the underlying
buffer-fill/flush operations on the underlying fd is unspecified. It's
really unfortunate that the kernel lies that procfs files are regular
files but doesn't give them regular-file semantics, but you really
need to use direct operations on the fd in the units the interface
requires, rather than stdio, to work with these files.

Rich