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util/nvmutil: block O_APPEND in prw()

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O_APPEND allows writes at EOF, thus breaking
positional read/write

Signed-off-by: Leah Rowe <leah@libreboot.org>
This commit is contained in:
Leah Rowe
2026-03-13 14:29:37 +00:00
parent e2f55af07f
commit 087c7a6ad2
1 changed files with 13 additions and 11 deletions
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24
util/nvmutil/nvmutil.c
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@@ -1557,17 +1557,6 @@ gbe_x_offset(size_t p, const char *f_op, const char *d_type,
* be used on sockets or pipes, because 0-byte
* reads are treated like fatal errors. This
* means that EOF is also considered fatal.
*
* WARNING: Do not use O_APPEND on open() when
* using this function. If you do, POSIX allows
* write() to ignore the current file offset and
* write at EOF, which means that our use of
* lseek in prw() does not guarantee writing at
* a specified offset. So if using IO_PWRITE or
* IO_PREAD, make sure not to pass a file descriptor
* with the O_APPEND flag. Alternatively, modify
* do_rw() to directly use pwrite() and pread()
* instead of prw().
*/
static ssize_t
rw_file_exact(int fd, uint8_t *mem, size_t len,
@@ -1666,6 +1655,7 @@ prw(int fd, void *mem, size_t nrw,
ssize_t r;
int saved_errno;
int prw_type;
int flags;
prw_type = rw_type ^ IO_PREAD;
@@ -1673,6 +1663,18 @@ prw(int fd, void *mem, size_t nrw,
|| (unsigned int)prw_type > IO_WRITE)
goto err_prw;
/*
* O_APPEND must not be used, because this
* allows POSIX write() to ignore the
* current write offset and write at EOF,
* which would therefore break pread/pwrite
*/
flags = fcntl(fd, F_GETFL);
if (flags == -1)
return -1;
if (flags & O_APPEND)
goto err_prw;
if ((off_orig = lseek_eintr(fd, (off_t)0, SEEK_CUR)) == (off_t)-1)
return -1;
if (lseek_eintr(fd, off, SEEK_SET) == (off_t)-1)