Re: [Xen-devel] [PATCH v4] tools/libxc: Implement writev_exact() in the same style as write_exact()

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2015-02-18 at 18:31 +0000, Andrew Cooper wrote:
> This implementation of writev_exact() will cope with an iovcnt greater than
> IOV_MAX because glibc will actually let this work anyway, and it is very
> useful not to have to work about this in the caller of writev_exact().  The

s/work/worry?

Will this still work with non-glibc libcs which do not cope with iovcnt
> IOV_MAX?

In fact looking at the code it's not clear what glibc is letting work
anyway, do you pass a count > IOV_MAX to writev? Doesn't look like it.


> caller is still required to ensure that the sum of iov_len's doesn't overflow
> a ssize_t.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
> CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> 
> ---
> v4:
>  * Allow this to compile in a stubdom environment.
> v3:
>  * Re-add adjustment for partial writes.
>  * Split min/max adjustment into separate patch.
> v2:
>  * Remove adjustment for partial writes of a specific iov[] entry.
> ---
>  tools/libxc/xc_private.c |   79 
> ++++++++++++++++++++++++++++++++++++++++++++++
>  tools/libxc/xc_private.h |   14 ++++++++
>  2 files changed, 93 insertions(+)
> 
> diff --git a/tools/libxc/xc_private.c b/tools/libxc/xc_private.c
> index df6cd9b..35c514a 100644
> --- a/tools/libxc/xc_private.c
> +++ b/tools/libxc/xc_private.c
> @@ -860,6 +860,85 @@ int write_exact(int fd, const void *data, size_t size)
>      return 0;
>  }
>  
> +#if defined(__MINIOS__)
> +/*
> + * MiniOS's libc doesn't know about writev(). Implement it as multiple 
> write()s.
> + */
> +int writev_exact(int fd, const struct iovec *iov, int iovcnt)
> +{
> +    int rc, i;
> +
> +    for ( i = 0; i < iovcnt; ++i )
> +    {
> +        rc = write_exact(fd, iov[i].iov_base, iov[i].iov_len);
> +        if ( rc )
> +            return rc;
> +    }
> +
> +    return 0;
> +}
> +#else
> +int writev_exact(int fd, const struct iovec *iov, int iovcnt)
> +{
> +    struct iovec *local_iov = NULL;
> +    int rc = 0, iov_idx = 0, saved_errno = 0;
> +    ssize_t len;
> +
> +    while ( iov_idx < iovcnt )
> +    {
> +        /* Skip over iov[] entries with 0 length. */
> +        while ( iov[iov_idx].iov_len == 0 )
> +            if ( ++iov_idx == iovcnt )
> +                goto out;
> +
> +        len = writev(fd, &iov[iov_idx], min(iovcnt - iov_idx, IOV_MAX));
> +        saved_errno = errno;
> +
> +        if ( (len == -1) && (errno == EINTR) )
> +            continue;
> +        if ( len <= 0 )
> +        {
> +            rc = -1;
> +            goto out;
> +        }
> +
> +        /* Check iov[] to see whether we had a partial or complete write. */
> +        while ( len > 0 && (iov_idx < iovcnt) )
> +        {
> +            if ( len >= iov[iov_idx].iov_len )
> +                len -= iov[iov_idx++].iov_len;
> +            else
> +            {
> +                /* Partial write of iov[iov_idx]. Copy iov so we can adjust
> +                 * element iov_idx and resubmit the rest. */
> +                if ( !local_iov )
> +                {
> +                    local_iov = malloc(iovcnt * sizeof(*iov));
> +                    if ( !local_iov )
> +                    {
> +                        saved_errno = ENOMEM;
> +                        goto out;
> +                    }
> +
> +                    iov = memcpy(local_iov, iov, iovcnt * sizeof(*iov));
> +                }
> +
> +                local_iov[iov_idx].iov_base += len;
> +                local_iov[iov_idx].iov_len  -= len;
> +                break;
> +            }
> +        }
> +    }
> +
> +    saved_errno = 0;
> +
> + out:
> +    free(local_iov);
> +    errno = saved_errno;
> +    return rc;
> +}
> +#endif
> +
>  int xc_ffs8(uint8_t x)
>  {
>      int i;
> diff --git a/tools/libxc/xc_private.h b/tools/libxc/xc_private.h
> index 45b8644..f74f7d7 100644
> --- a/tools/libxc/xc_private.h
> +++ b/tools/libxc/xc_private.h
> @@ -42,6 +42,19 @@
>  #define VALGRIND_MAKE_MEM_UNDEFINED(addr, len) /* addr, len */
>  #endif
>  
> +#if defined(__MINIOS__)
> +/*
> + * MiniOS's libc doesn't know about sys/uio.h or writev().
> + * Declare enough of sys/uio.h to compile.
> + */
> +struct iovec {
> +    void *iov_base;
> +    size_t iov_len;
> +};
> +#else
> +#include <sys/uio.h>
> +#endif
> +
>  #define DECLARE_HYPERCALL privcmd_hypercall_t hypercall
>  #define DECLARE_DOMCTL struct xen_domctl domctl
>  #define DECLARE_SYSCTL struct xen_sysctl sysctl
> @@ -395,6 +408,7 @@ int xc_add_mmu_update(xc_interface *xch, struct xc_mmu 
> *mmu,
>  /* Return 0 on success; -1 on error setting errno. */
>  int read_exact(int fd, void *data, size_t size); /* EOF => -1, errno=0 */
>  int write_exact(int fd, const void *data, size_t size);
> +int writev_exact(int fd, const struct iovec *iov, int iovcnt);
>  
>  int xc_ffs8(uint8_t x);
>  int xc_ffs16(uint16_t x);



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