Stas,
I have attempted to document the SS_AUTODISARM feature that you added
in Linux 4.7.
Could you please take a look at the SS_AUTODISARM pieces in the
sigaltstack() man page below? There is also one FIXME that I would
like help with.
It seems to me that the API has become rather odd now. It is no longer
possible to simply check whether code is executing on an alternative
stack by using
sigaltstack(NULL, &old_ss);
if (old_ss.ss_flags & SS_ONSTACK)
....
Thanks,
Michael
NAME
sigaltstack - set and/or get signal stack context
SYNOPSIS
#include <signal.h>
int sigaltstack(const stack_t *ss, stack_t *old_ss);
Feature Test Macro Requirements for glibc (see fea‐
ture_test_macros(7)):
sigaltstack():
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE
DESCRIPTION
sigaltstack() allows a process to define a new alternate signal
stack and/or retrieve the state of an existing alternate signal
stack. An alternate signal stack is used during the execution
of a signal handler if the establishment of that handler (see
sigaction(2)) requested it.
The normal sequence of events for using an alternate signal
stack is the following:
1. Allocate an area of memory to be used for the alternate sig‐
nal stack.
2. Use sigaltstack() to inform the system of the existence and
location of the alternate signal stack.
3. When establishing a signal handler using sigaction(2),
inform the system that the signal handler should be executed
on the alternate signal stack by specifying the SA_ONSTACK
flag.
The ss argument is used to specify a new alternate signal
stack, while the old_ss argument is used to retrieve informa‐
tion about the currently established signal stack. If we are
interested in performing just one of these tasks, then the
other argument can be specified as NULL.
The stack_t type used to type the arguments of this function is
defined as follows:
typedef struct {
void *ss_sp; /* Base address of stack */
int ss_flags; /* Flags */
size_t ss_size; /* Number of bytes in stack */
} stack_t;
To establish a new alternate signal stack, the fields of this
structure are set as follows:
ss.ss_flags
This field contains either 0, or the following flag:
SS_AUTODISARM (since Linux 4.7)
Clear the alternate signal stack settings on
entry to the signal handler. When the signal
handler returns, the previous alternate signal
stack settings are restored.
This flag was added in order make it safe to
switch away from the signal handler with swapcon‐
text(3). Without this flag, a subsequently han‐
dled signal will corrupt the state of the
switched-away signal handler. On kernels where
this flag is not supported, sigaltstack() fails
with the error EINVAL when this flag is supplied.
ss.ss_sp
This field specifies the starting address of the stack.
When a signal handler is invoked on the alternate stack,
the kernel automatically aligns the address given in
ss.ss_sp to a suitable address boundary for the underly‐
ing hardware architecture.
ss.ss_size
This field specifies the size of the stack. The con‐
stant SIGSTKSZ is defined to be large enough to cover
the usual size requirements for an alternate signal
stack, and the constant MINSIGSTKSZ defines the minimum
size required to execute a signal handler.
To disable an existing stack, specify ss.ss_flags as SS_DIS‐
ABLE. In this case, the kernel ignores any other flags in
ss.ss_flags and the remaining fields in ss.
If old_ss is not NULL, then it is used to return information
about the alternate signal stack which was in effect prior to
the call to sigaltstack(). The old_ss.ss_sp and old_ss.ss_size
fields return the starting address and size of that stack. The
old_ss.ss_flags may return either of the following values:
SS_ONSTACK
The process is currently executing on the alternate sig‐
nal stack. (Note that it is not possible to change the
alternate signal stack if the process is currently exe‐
cuting on it.)
SS_DISABLE
The alternate signal stack is currently disabled.
Alternatively, this value is returned if the process is
currently executing on an alternate signal stack that
was established using the SS_AUTODISARM flag. In this
case, it is safe to switch away from the signal handler
with swapcontext(3). It is also possible to set up a
different alternative signal stack using a further call
to sigaltstack().
┌─────────────────────────────────────────────────────┐
│FIXME │
├─────────────────────────────────────────────────────┤
│Was it intended that one can set up a different │
│alternative signal stack in this scenario? (In pass‐ │
│ing, if one does this, the sigaltstack(NULL, │
│&old_ss) now returns old_ss.ss_flags==SS_AUTODISARM │
│rather than old_ss.ss_flags==SS_DISABLE. The API │
│design here seems confusing... │
└─────────────────────────────────────────────────────┘
SS_AUTODISARM
The alternate signal stack has been marked to be
autodisarmed as described above.
By specifying ss as NULL, and old_ss as a non-NULL value, one
can obtain the current settings for the alternate signal stack
without changing them.
RETURN VALUE
sigaltstack() returns 0 on success, or -1 on failure with errno
set to indicate the error.
ERRORS
EFAULT Either ss or old_ss is not NULL and points to an area
outside of the process's address space.
EINVAL ss is not NULL and the ss_flags field contains an
invalid flag.
ENOMEM The specified size of the new alternate signal stack
ss.ss_size was less than MINSTKSZ.
EPERM An attempt was made to change the alternate signal stack
while it was active (i.e., the process was already exe‐
cuting on the current alternate signal stack).
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────┼───────────────┼─────────┤
│sigaltstack() │ Thread safety │ MT-Safe │
└──────────────┴───────────────┴─────────┘
CONFORMING TO
POSIX.1-2001, POSIX.1-2009, SUSv2, SVr4.
The SS_AUTODISARM flag is a Linux extension.
NOTES
The most common usage of an alternate signal stack is to handle
the SIGSEGV signal that is generated if the space available for
the normal process stack is exhausted: in this case, a signal
handler for SIGSEGV cannot be invoked on the process stack; if
we wish to handle it, we must use an alternate signal stack.
Establishing an alternate signal stack is useful if a process
expects that it may exhaust its standard stack. This may
occur, for example, because the stack grows so large that it
encounters the upwardly growing heap, or it reaches a limit
established by a call to setrlimit(RLIMIT_STACK, &rlim). If
the standard stack is exhausted, the kernel sends the process a
SIGSEGV signal. In these circumstances the only way to catch
this signal is on an alternate signal stack.
On most hardware architectures supported by Linux, stacks grow
downward. sigaltstack() automatically takes account of the
direction of stack growth.
Functions called from a signal handler executing on an alter‐
nate signal stack will also use the alternate signal stack.
(This also applies to any handlers invoked for other signals
while the process is executing on the alternate signal stack.)
Unlike the standard stack, the system does not automatically
extend the alternate signal stack. Exceeding the allocated
size of the alternate signal stack will lead to unpredictable
results.
A successful call to execve(2) removes any existing alternate
signal stack. A child process created via fork(2) inherits a
copy of its parent's alternate signal stack settings.
sigaltstack() supersedes the older sigstack() call. For back‐
ward compatibility, glibc also provides sigstack(). All new
applications should be written using sigaltstack().
History
4.2BSD had a sigstack() system call. It used a slightly dif‐
ferent struct, and had the major disadvantage that the caller
had to know the direction of stack growth.
EXAMPLE
The following code segment demonstrates the use of sigalt‐
stack():
stack_t ss;
ss.ss_sp = malloc(SIGSTKSZ);
if (ss.ss_sp == NULL)
/* Handle error */;
ss.ss_size = SIGSTKSZ;
ss.ss_flags = 0;
if (sigaltstack(&ss, NULL) == -1)
/* Handle error */;
BUGS
In the lead up to the development of the Linux 2.4 kernel,
someone got confused and allowed the kernel to accept
SS_ONSTACK in ss.ss_flags, which results behavior that is the
same as when ss_flags is 0. On other implementations, and
according to POSIX.1, SS_ONSTACK appears only as a reported
flag in old_ss.ss_flags. There is no need ever to specify this
flag in ss.ss_flags.
SEE ALSO
execve(2), setrlimit(2), sigaction(2), siglongjmp(3),
sigsetjmp(3), signal(7)
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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