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login: Use pread64 in utmp implementation 

This reduces the possible error scenarios considerably because
no longer can file seek fail, leaving the file descriptor in an
inconsistent state and out of sync with the cache.

As a result, it is possible to avoid setting file_offset to -1
to make an error persistent.  Instead, subsequent calls will retry
the operation and report any errors returned by the kernel.

This change also avoids reading the file from the start if pututline
is called multiple times, to work around lock acquisition failures
due to timeouts.

Change-Id: If21ea0c162c38830a89331ea93cddec14c0974de
This commit is contained in:
Florian Weimer 2019-11-12 12:25:49 +01:00
parent ca136bb0a3
commit d4625a19fe
1 changed files with 96 additions and 73 deletions
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169
login/utmp_file.c
View File

@ -162,12 +162,35 @@ maybe_setutent (void)
return file_fd >= 0 || __libc_setutent ();
}
/* Reads the entry at file_offset, storing it in last_entry and
updating file_offset on success. Returns -1 for a read error, 0
for EOF, and 1 for a successful read. last_entry and file_offset
are only updated on a successful and complete read. */
static ssize_t
read_last_entry (void)
{
struct utmp buffer;
ssize_t nbytes = __pread64_nocancel (file_fd, &buffer, sizeof (buffer),
file_offset);
if (nbytes < 0)
return -1;
else if (nbytes != sizeof (buffer))
/* Assume EOF. */
return 0;
else
{
last_entry = buffer;
file_offset += sizeof (buffer);
return 1;
}
}
int
__libc_getutent_r (struct utmp *buffer, struct utmp **result)
{
ssize_t nbytes;
int saved_errno = errno;
if (!maybe_setutent () || file_offset == -1l)
if (!maybe_setutent ())
{
/* Not available. */
*result = NULL;
@ -175,25 +198,22 @@ __libc_getutent_r (struct utmp *buffer, struct utmp **result)
}
if (try_file_lock (file_fd, F_RDLCK))
nbytes = 0;
else
{
/* Read the next entry. */
nbytes = __read_nocancel (file_fd, &last_entry, sizeof (struct utmp));
file_unlock (file_fd);
}
return -1;
if (nbytes != sizeof (struct utmp))
ssize_t nbytes = read_last_entry ();
file_unlock (file_fd);
if (nbytes <= 0) /* Read error or EOF. */
{
if (nbytes != 0)
file_offset = -1l;
if (nbytes == 0)
/* errno should be unchanged to indicate success. A premature
EOF is treated like an EOF (missing complete record at the
end). */
__set_errno (saved_errno);
*result = NULL;
return -1;
}
/* Update position pointer. */
file_offset += sizeof (struct utmp);
memcpy (buffer, &last_entry, sizeof (struct utmp));
*result = buffer;
@ -209,15 +229,15 @@ internal_getut_nolock (const struct utmp *id)
{
while (1)
{
/* Read the next entry. */
if (__read_nocancel (file_fd, &last_entry, sizeof (struct utmp))
!= sizeof (struct utmp))
ssize_t nbytes = read_last_entry ();
if (nbytes < 0)
return -1;
if (nbytes == 0)
{
/* End of file reached. */
__set_errno (ESRCH);
file_offset = -1l;
return -1;
}
file_offset += sizeof (struct utmp);
if (matches_last_entry (id))
break;
@ -249,7 +269,7 @@ int
__libc_getutid_r (const struct utmp *id, struct utmp *buffer,
struct utmp **result)
{
if (!maybe_setutent () || file_offset == -1l)
if (!maybe_setutent ())
{
*result = NULL;
return -1;
@ -276,7 +296,7 @@ int
__libc_getutline_r (const struct utmp *line, struct utmp *buffer,
struct utmp **result)
{
if (!maybe_setutent () || file_offset == -1l)
if (!maybe_setutent ())
{
*result = NULL;
return -1;
@ -290,16 +310,21 @@ __libc_getutline_r (const struct utmp *line, struct utmp *buffer,
while (1)
{
/* Read the next entry. */
if (__read_nocancel (file_fd, &last_entry, sizeof (struct utmp))
!= sizeof (struct utmp))
ssize_t nbytes = read_last_entry ();
if (nbytes < 0)
{
__set_errno (ESRCH);
file_offset = -1l;
file_unlock (file_fd);
*result = NULL;
goto unlock_return;
return -1;
}
if (nbytes == 0)
{
/* End of file reached. */
file_unlock (file_fd);
__set_errno (ESRCH);
*result = NULL;
return -1;
}
file_offset += sizeof (struct utmp);
/* Stop if we found a user or login entry. */
if ((last_entry.ut_type == USER_PROCESS
@ -309,20 +334,18 @@ __libc_getutline_r (const struct utmp *line, struct utmp *buffer,
break;
}
file_unlock (file_fd);
memcpy (buffer, &last_entry, sizeof (struct utmp));
*result = buffer;
unlock_return:
file_unlock (file_fd);
return ((*result == NULL) ? -1 : 0);
return 0;
}
struct utmp *
__libc_pututline (const struct utmp *data)
{
if (!maybe_setutent () || file_offset == -1l)
if (!maybe_setutent ())
return NULL;
struct utmp *pbuf;
@ -337,8 +360,7 @@ __libc_pututline (const struct utmp *data)
if (new_fd == -1)
return NULL;
if (__lseek64 (new_fd, __lseek64 (file_fd, 0, SEEK_CUR), SEEK_SET) == -1
|| __dup2 (new_fd, file_fd) < 0)
if (__dup2 (new_fd, file_fd) < 0)
{
__close_nocancel_nostatus (new_fd);
return NULL;
@ -355,69 +377,70 @@ __libc_pututline (const struct utmp *data)
bool found = false;
if (matches_last_entry (data))
{
if (__lseek64 (file_fd, file_offset, SEEK_SET) < 0)
/* Read back the entry under the write lock. */
file_offset -= sizeof (last_entry);
ssize_t nbytes = read_last_entry ();
if (nbytes < 0)
{
file_unlock (file_fd);
return NULL;
}
if (__read_nocancel (file_fd, &last_entry, sizeof (last_entry))
!= sizeof (last_entry))
{
if (__lseek64 (file_fd, file_offset, SEEK_SET) < 0)
{
file_unlock (file_fd);
return NULL;
}
found = false;
}
if (nbytes == 0)
/* End of file reached. */
found = false;
else
found = matches_last_entry (data);
}
if (!found)
/* Search forward for the entry. */
found = internal_getut_nolock (data) >= 0;
off64_t write_offset;
if (!found)
{
/* We append the next entry. */
file_offset = __lseek64 (file_fd, 0, SEEK_END);
if (file_offset % sizeof (struct utmp) != 0)
{
file_offset -= file_offset % sizeof (struct utmp);
__ftruncate64 (file_fd, file_offset);
write_offset = __lseek64 (file_fd, 0, SEEK_END);
if (__lseek64 (file_fd, 0, SEEK_END) < 0)
{
pbuf = NULL;
goto unlock_return;
}
}
/* Round down to the next multiple of the entry size. This
ensures any partially-written record is overwritten by the
new record. */
write_offset = (write_offset / sizeof (struct utmp)
* sizeof (struct utmp));
}
else
{
/* We replace the just read entry. */
file_offset -= sizeof (struct utmp);
__lseek64 (file_fd, file_offset, SEEK_SET);
}
/* Overwrite last_entry. */
write_offset = file_offset - sizeof (struct utmp);
/* Write the new data. */
if (__write_nocancel (file_fd, data, sizeof (struct utmp))
!= sizeof (struct utmp))
ssize_t nbytes;
if (__lseek64 (file_fd, write_offset, SEEK_SET) < 0
|| (nbytes = __write_nocancel (file_fd, data, sizeof (struct utmp))) < 0)
{
/* There is no need to recover the file position because all
reads use pread64, and any future write is preceded by
another seek. */
file_unlock (file_fd);
return NULL;
}
if (nbytes != sizeof (struct utmp))
{
/* If we appended a new record this is only partially written.
Remove it. */
if (!found)
(void) __ftruncate64 (file_fd, file_offset);
pbuf = NULL;
}
else
{
file_offset += sizeof (struct utmp);
pbuf = (struct utmp *) data;
(void) __ftruncate64 (file_fd, write_offset);
file_unlock (file_fd);
/* Assume that the write failure was due to missing disk
space. */
__set_errno (ENOSPC);
return NULL;
}
unlock_return:
file_unlock (file_fd);
file_offset = write_offset + sizeof (struct utmp);
pbuf = (struct utmp *) data;
return pbuf;
}