libc/stdlib/malloc.3

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.\"	$OpenBSD: malloc.3,v 1.128 2021/04/09 06:04:15 otto Exp $
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.Dd $Mdocdate: April 9 2021 $
.Dt MALLOC 3
.Os
.Sh NAME
.Nm malloc ,
.Nm calloc ,
.Nm realloc ,
.Nm free ,
.Nm reallocarray ,
.Nm recallocarray ,
.Nm freezero ,
.Nm aligned_alloc ,
.Nm malloc_conceal ,
.Nm calloc_conceal
.Nd memory allocation and deallocation
.Sh SYNOPSIS
.In stdlib.h
.Ft void *
.Fn malloc "size_t size"
.Ft void *
.Fn calloc "size_t nmemb" "size_t size"
.Ft void *
.Fn realloc "void *ptr" "size_t size"
.Ft void
.Fn free "void *ptr"
.Ft void *
.Fn reallocarray "void *ptr" "size_t nmemb" "size_t size"
.Ft void *
.Fn recallocarray "void *ptr" "size_t oldnmemb" "size_t nmemb" "size_t size"
.Ft void
.Fn freezero "void *ptr" "size_t size"
.Ft void *
.Fn aligned_alloc "size_t alignment" "size_t size"
.Ft void *
.Fn malloc_conceal "size_t size"
.Ft void *
.Fn calloc_conceal "size_t nmemb" "size_t size"
.Vt char *malloc_options ;
.Sh DESCRIPTION
The standard functions
.Fn malloc ,
.Fn calloc ,
and
.Fn realloc
allocate
.Em objects ,
regions of memory to store values.
The
.Fn malloc
function allocates uninitialized space for an object of
the specified
.Fa size .
.Fn malloc
maintains multiple lists of free objects according to size, allocating
from the appropriate list or requesting memory from the kernel.
The allocated space is suitably aligned (after possible pointer coercion) for
storage of any type of object.
.Pp
The
.Fn calloc
function allocates space for an array of
.Fa nmemb
objects, each of the specified
.Fa size .
The space is initialized to zero.
.Pp
The
.Fn realloc
function changes the size of the object pointed to by
.Fa ptr
to
.Fa size
bytes and returns a pointer to the (possibly moved) object.
If
.Fa ptr
is not
.Dv NULL ,
it must be a pointer returned by an earlier call to an allocation or
reallocation function that was not freed in between.
The contents of the object are unchanged up to the lesser
of the new and old sizes.
If the new size is larger, the value of the newly allocated portion
of the object is indeterminate and uninitialized.
If the space cannot be allocated, the object
pointed to by
.Fa ptr
is unchanged.
If
.Fa ptr
is
.Dv NULL ,
.Fn realloc
behaves like
.Fn malloc
and allocates a new object.
.Pp
The
.Fn free
function causes the space pointed to by
.Fa ptr
to be either placed on a list of free blocks to make it available for future
allocation or, when appropriate, to be returned to the kernel using
.Xr munmap 2 .
If
.Fa ptr
is
.Dv NULL ,
no action occurs.
If
.Fa ptr
was previously freed by
.Fn free
or a reallocation function,
the behavior is undefined and the double free is a security concern.
.Pp
Designed for safe allocation of arrays,
the
.Fn reallocarray
function is similar to
.Fn realloc
except it operates on
.Fa nmemb
members of size
.Fa size
and checks for integer overflow in the calculation
.Fa nmemb
*
.Fa size .
.Pp
Used for the allocation of memory holding sensitive data,
the
.Fn recallocarray
and
.Fn freezero
functions guarantee that memory becoming unallocated is explicitly
.Em discarded ,
meaning pages of memory are disposed via
.Xr munmap 2
and cached free objects are cleared with
.Xr explicit_bzero 3 .
.Pp
The
.Fn recallocarray
function is similar to
.Fn reallocarray
except it ensures newly allocated memory is cleared similar to
.Fn calloc .
If
.Fa ptr
is
.Dv NULL ,
.Fa oldnmemb
is ignored and the call is equivalent to
.Fn calloc .
If
.Fa ptr
is not
.Dv NULL ,
.Fa oldnmemb
must be a value such that
.Fa oldnmemb
*
.Fa size
is the size of the earlier allocation that returned
.Fa ptr ,
otherwise the behavior is undefined.
.Pp
The
.Fn freezero
function is similar to the
.Fn free
function except it ensures memory is explicitly discarded.
If
.Fa ptr
is
.Dv NULL ,
no action occurs.
If
.Fa ptr
is not
.Dv NULL ,
the
.Fa size
argument must be equal to or smaller than the size of the earlier allocation
that returned
.Fa ptr .
.Fn freezero
guarantees the memory range starting at
.Fa ptr
with length
.Fa size
is discarded while deallocating the whole object originally allocated.
.Pp
The
.Fn aligned_alloc
function allocates
.Fa size
bytes of memory such that the allocation's base address is a multiple of
.Fa alignment .
The requested
.Fa alignment
must be a power of 2.
If
.Fa size
is not a multiple of
.Fa alignment ,
behavior is undefined.
.Pp
The
.Fn malloc_conceal
and
.Fn calloc_conceal
functions behave the same as
.Fn malloc
and
.Fn calloc
respectively,
with the exception that the allocation returned is marked with the
.Dv MAP_CONCEAL
.Xr mmap 2
flag and calling
.Fn free
on the allocation will discard the contents explicitly.
A reallocation of a concealed allocation will leave these properties intact.
.Sh MALLOC OPTIONS
Upon the first call to the
.Fn malloc
family of functions, an initialization sequence inspects the
value of the
.Va vm.malloc_conf
.Xr sysctl 2 ,
next checks the environment for a variable called
.Ev MALLOC_OPTIONS ,
and finally looks at the global variable
.Va malloc_options
in the program.
Each is scanned for the flags documented below.
Unless otherwise noted uppercase means on, lowercase means off.
During initialization, flags occurring later modify the behaviour
that was requested by flags processed earlier.
.Bl -tag -width indent
.It Cm C
.Dq Canaries .
Add canaries at the end of allocations in order to detect
heap overflows.
The canary's content is checked when
.Nm free
is called.
If it has been corrupted, the process is aborted.
.It Cm D
.Dq Dump .
.Fn malloc
will dump statistics to the file
.Pa ./malloc.out ,
if it already exists,
at exit.
This option requires the library to have been compiled with -DMALLOC_STATS in
order to have any effect.
.It Cm F
.Dq Freecheck .
Enable more extensive double free and use after free detection.
All chunks in the delayed free list will be checked for double frees.
Unused pages on the freelist are read and write protected to
cause a segmentation fault upon access.
.It Cm G
.Dq Guard .
Enable guard pages.
Each page size or larger allocation is followed by a guard page that will
cause a segmentation fault upon any access.
.It Cm J
.Dq More junking .
Increase the junk level by one if it is smaller than 2.
.It Cm j
.Dq Less junking .
Decrease the junk level by one if it is larger than 0.
Junking writes some junk bytes into the area allocated.
Junk is bytes of 0xdb when allocating;
freed chunks are filled with 0xdf.
By default the junk level is 1: after free,
small chunks are completely junked;
for pages the first part is junked.
After a delay,
the filling pattern is validated and the process is aborted if the pattern
was modified.
For junk level 2, junking is done on allocation as well and without size
restrictions.
If the junk level is zero, no junking is performed.
.It Cm R
.Dq realloc .
Always reallocate when
.Fn realloc
is called, even if the initial allocation was big enough.
.\".Pp
.\".It Cm U
.\".Dq utrace .
.\"Generate entries for
.\".Xr ktrace 1
.\"for all operations.
.\"Consult the source for this one.
.It Cm S
.\" Malloc option S is vaguely documented on purpose.
Enable all options suitable for security auditing.
.It Cm U
.Dq Free unmap .
Enable use after free protection for larger allocations.
Unused pages on the freelist are read and write protected to
cause a segmentation fault upon access.
.It Cm X
.Dq xmalloc .
Rather than return failure,
.Xr abort 3
the program with a diagnostic message on stderr.
It is the intention that this option be set at compile time by
including in the source:
.Bd -literal -offset indent
extern char *malloc_options;
malloc_options = "X";
.Ed
.Pp
Note that this will cause code that is supposed to handle
out-of-memory conditions gracefully to abort instead.
.It Cm <
.Dq Halve the cache size .
Decrease the size of the free page cache by a factor of two.
.It Cm >
.Dq Double the cache size .
Increase the size of the free page cache by a factor of two.
.El
.Pp
If a program changes behavior if any of these options (except
.Cm X )
are used,
it is buggy.
.Pp
The default size of the cache is 64 single page allocations.
It also caches a number of larger regions.
Multi-threaded programs use multiple pools.
.Sh RETURN VALUES
Upon successful completion, the allocation functions
return a pointer to the allocated space; otherwise,
.Dv NULL
is returned and
.Va errno
is set to
.Er ENOMEM .
The function
.Fn aligned_alloc
returns
.Dv NULL
and sets
.Va errno
to
.Er EINVAL
if
.Fa alignment
is not a power of 2.
.Pp
If
.Fa nmemb
or
.Fa size
is equal to 0, a unique pointer to an access protected,
zero sized object is returned.
Access via this pointer will generate a
.Dv SIGSEGV
exception.
.Pp
If multiplying
.Fa nmemb
and
.Fa size
results in integer overflow,
.Fn calloc ,
.Fn reallocarray
and
.Fn recallocarray
return
.Dv NULL
and set
.Va errno
to
.Er ENOMEM .
.Pp
If
.Fa ptr
is not
.Dv NULL
and multiplying
.Fa oldnmemb
and
.Fa size
results in integer overflow
.Fn recallocarray
returns
.Dv NULL
and sets
.Va errno
to
.Er EINVAL .
.Sh IDIOMS
Consider
.Fn calloc
or the extensions
.Fn reallocarray
and
.Fn recallocarray
when there is multiplication in the
.Fa size
argument of
.Fn malloc
or
.Fn realloc .
For example, avoid this common idiom as it may lead to integer overflow:
.Bd -literal -offset indent
if ((p = malloc(num * size)) == NULL)
	err(1, NULL);
.Ed
.Pp
A drop-in replacement is the
.Ox
extension
.Fn reallocarray :
.Bd -literal -offset indent
if ((p = reallocarray(NULL, num, size)) == NULL)
	err(1, NULL);
.Ed
.Pp
Alternatively,
.Fn calloc
may be used at the cost of initialization overhead.
.Pp
When using
.Fn realloc ,
be careful to avoid the following idiom:
.Bd -literal -offset indent
size += 50;
if ((p = realloc(p, size)) == NULL)
	return (NULL);
.Ed
.Pp
Do not adjust the variable describing how much memory has been allocated
until the allocation has been successful.
This can cause aberrant program behavior if the incorrect size value is used.
In most cases, the above sample will also result in a leak of memory.
As stated earlier, a return value of
.Dv NULL
indicates that the old object still remains allocated.
Better code looks like this:
.Bd -literal -offset indent
newsize = size + 50;
if ((newp = realloc(p, newsize)) == NULL) {
	free(p);
	p = NULL;
	size = 0;
	return (NULL);
}
p = newp;
size = newsize;
.Ed
.Pp
As with
.Fn malloc ,
it is important to ensure the new size value will not overflow;
i.e. avoid allocations like the following:
.Bd -literal -offset indent
if ((newp = realloc(p, num * size)) == NULL) {
	...
.Ed
.Pp
Instead, use
.Fn reallocarray :
.Bd -literal -offset indent
if ((newp = reallocarray(p, num, size)) == NULL) {
	...
.Ed
.Pp
Calling
.Fn realloc
with a
.Dv NULL
.Fa ptr
is equivalent to calling
.Fn malloc .
Instead of this idiom:
.Bd -literal -offset indent
if (p == NULL)
	newp = malloc(newsize);
else
	newp = realloc(p, newsize);
.Ed
.Pp
Use the following:
.Bd -literal -offset indent
newp = realloc(p, newsize);
.Ed
.Pp
The
.Fn recallocarray
function should be used for resizing objects containing sensitive data like
keys.
To avoid leaking information,
it guarantees memory is cleared before placing it on the internal free list.
Deallocation of such an object should be done by calling
.Fn freezero .
.Sh ENVIRONMENT
.Bl -tag -width "MALLOC_OPTIONS"
.It Ev MALLOC_OPTIONS
String of option flags.
.El
.Sh EXAMPLES
If
.Fn malloc
must be used with multiplication, be sure to test for overflow:
.Bd -literal -offset indent
size_t num, size;
\&...

/* Check for size_t overflow */
if (size && num > SIZE_MAX / size)
	errc(1, EOVERFLOW, "overflow");

if ((p = malloc(num * size)) == NULL)
	err(1, NULL);
.Ed
.Pp
The above test is not sufficient in all cases.
For example, multiplying ints requires a different set of checks:
.Bd -literal -offset indent
int num, size;
\&...

/* Avoid invalid requests */
if (size < 0 || num < 0)
	errc(1, EOVERFLOW, "overflow");

/* Check for signed int overflow */
if (size && num > INT_MAX / size)
	errc(1, EOVERFLOW, "overflow");

if ((p = malloc(num * size)) == NULL)
	err(1, NULL);
.Ed
.Pp
Assuming the implementation checks for integer overflow as
.Ox
does, it is much easier to use
.Fn calloc ,
.Fn reallocarray ,
or
.Fn recallocarray .
.Pp
The above examples could be simplified to:
.Bd -literal -offset indent
if ((p = reallocarray(NULL, num, size)) == NULL)
	err(1, NULL);
.Ed
.Pp
or at the cost of initialization:
.Bd -literal -offset indent
if ((p = calloc(num, size)) == NULL)
	err(1, NULL);
.Ed
.Pp
Set a systemwide reduction of the cache to a quarter of the
default size and use guard pages:
.Pp
.Dl # sysctl vm.malloc_conf='G<<'
.Sh DIAGNOSTICS
If any of the functions detect an error condition,
a message will be printed to file descriptor
2 (not using stdio).
Errors will result in the process being aborted.
.Pp
Here is a brief description of the error messages and what they mean:
.Bl -tag -width Ds
.It Dq out of memory
If the
.Cm X
option is specified it is an error for the allocation functions
to return
.Dv NULL .
.It Dq bogus pointer (double free?)
An attempt to
.Fn free
or
reallocate an unallocated pointer was made.
.It Dq chunk is already free
There was an attempt to free a chunk that had already been freed.
.It Dq write after free
A chunk has been modified after it was freed.
.It Dq modified chunk-pointer
The pointer passed to
.Fn free
or a reallocation function has been modified.
.It Dq chunk canary corrupted address offset@length
A byte after the requested size has been overwritten,
indicating a heap overflow.
The offset at which corruption was detected is printed before the @,
and the requested length of the allocation after the @.
.It Dq recorded old size oldsize != size
.Fn recallocarray
has detected that the given old size does not equal the recorded size in its
meta data.
Enabling option
.Cm C
allows
.Fn recallocarray
to catch more of these cases.
.It Dq recursive call
An attempt was made to call recursively into these functions, i.e., from a
signal handler.
This behavior is not supported.
In particular, signal handlers should
.Em not
use any of the
.Fn malloc
functions nor utilize any other functions which may call
.Fn malloc
(e.g.,
.Xr stdio 3
routines).
.It Dq unknown char in MALLOC_OPTIONS
We found something we didn't understand.
.It any other error
.Fn malloc
detected an internal error;
consult sources and/or wizards.
.El
.Sh SEE ALSO
.Xr brk 2 ,
.Xr mmap 2 ,
.Xr munmap 2 ,
.Xr sysctl 2 ,
.Xr alloca 3 ,
.Xr getpagesize 3 ,
.Xr posix_memalign 3
.Sh STANDARDS
The
.Fn malloc ,
.Fn calloc ,
.Fn realloc ,
and
.Fn free
functions conform to
.St -ansiC .
The
.Fn aligned_alloc
function conforms to
.St -isoC-2011 .
.Pp
If
.Fa nmemb
or
.Fa size
are 0, the return value is implementation defined;
other conforming implementations may return
.Dv NULL
in this case.
.Pp
The
.Ev MALLOC_OPTIONS
environment variable, the
.Va vm.malloc_conf
sysctl and the
.Sx DIAGNOSTICS
output are extensions to the standard.
.Sh HISTORY
A
.Fn free
internal kernel function and a predecessor to
.Fn malloc ,
.Fn alloc ,
first appeared in
.At v1 .
C library functions
.Fn alloc
and
.Fn free
appeared in
.At v6 .
The functions
.Fn malloc ,
.Fn calloc ,
and
.Fn realloc
first appeared in
.At v7 .
.Pp
A new implementation by Chris Kingsley was introduced in
.Bx 4.2 ,
followed by a complete rewrite by Poul-Henning Kamp which appeared in
.Fx 2.2
and was included in
.Ox 2.0 .
These implementations were all
.Xr sbrk 2
based.
In
.Ox 3.8 ,
Thierry Deval rewrote
.Nm
to use the
.Xr mmap 2
system call,
making the page addresses returned by
.Nm
random.
A rewrite by Otto Moerbeek introducing a new central data structure and more
randomization appeared in
.Ox 4.4 .
.Pp
The
.Fn reallocarray
function appeared in
.Ox 5.6 .
The
.Fn recallocarray
function appeared in
.Ox 6.1 .
The
.Fn freezero
function appeared in
.Ox 6.2 .
The
.Fn aligned_alloc
function appeared in
.Ox 6.5 .
The
.Fn malloc_conceal
and
.Fn calloc_conceal
functions appeared in
.Ox 6.6 .
.Sh CAVEATS
When using
.Fn malloc ,
be wary of signed integer and
.Vt size_t
overflow especially when there is multiplication in the
.Fa size
argument.
.Pp
Signed integer overflow will cause undefined behavior which compilers
typically handle by wrapping back around to negative numbers.
Depending on the input, this can result in allocating more or less
memory than intended.
.Pp
An unsigned overflow has defined behavior which will wrap back around and
return less memory than intended.
.Pp
A signed or unsigned integer overflow is a
.Em security
risk if less memory is returned than intended.
Subsequent code may corrupt the heap by writing beyond the memory that was
allocated.
An attacker may be able to leverage this heap corruption to execute arbitrary
code.
.Pp
Consider using
.Fn calloc ,
.Fn reallocarray
or
.Fn recallocarray
instead of using multiplication in
.Fn malloc
and
.Fn realloc
to avoid these problems on
.Ox .