@@ -38,6 +38,11 @@ Major new features:
and the wfN format length modifiers for arguments pointing to types
int_fastN_t or uint_fastN_t, as specified in draft ISO C2X.
+* A new tunable, glibc.mem.decorate_maps, can be used to add additional
+ information on underlying memory allocated by the glibc (for instance,
+ on thread stack created by pthread_create or memory allocated by
+ malloc).
+
Deprecated and removed features, and other changes affecting compatibility:
* The ldconfig program now skips file names containing ';' or ending in
@@ -3023,5 +3023,5 @@ $(objpfx)tst-dlclose-lazy.out: \
$(objpfx)tst-decorate-maps: $(shared-thread-library)
tst-decorate-maps-ENV = \
- GLIBC_TUNABLES=glibc.malloc.arena_max=8:glibc.malloc.mmap_threshold=1024
+ GLIBC_TUNABLES=glibc.malloc.arena_max=8:glibc.malloc.mmap_threshold=1024:glibc.mem.decorate_maps=1
tst-decorate-maps-ARGS = 8
@@ -160,6 +160,11 @@ glibc {
maxval: 255
security_level: SXID_IGNORE
}
+ decorate_maps {
+ type: INT_32
+ minval: 0
+ maxval: 1
+ }
}
rtld {
@@ -653,6 +653,18 @@ support in the kernel if this tunable has any non-zero value.
The default value is @samp{0}, which disables all memory tagging.
@end deftp
+@deftp Tunable glibc.mem.decorate_maps
+If the kernel supports naming anonymous virtual memory areas (since
+Linux version 5.17), this tunable can be used to control whether
+@theglibc{} decorates the underlying memory obtained from operating
+system with a string describing its usage (for instance, on the thread
+stack created by @code{ptthread_create} or memory allocated by
+@code{malloc}).
+
+This tunable takes a value of 0 and 1, where 1 enables the feature.
+The default value is @samp{0}, which disables the decoration.
+@end deftp
+
@node gmon Tunables
@section gmon Tunables
@cindex gmon tunables
@@ -20,6 +20,7 @@
#include <setvmaname.h>
#include <sys/prctl.h>
#include <sysdep.h>
+#include <elf/dl-tunables.h>
/* If PR_SET_VMA_ANON_NAME is not supported by the kernel, prctl returns
EINVAL. However, it also returns the same error for invalid argument.
@@ -30,6 +31,9 @@
void
__set_vma_name (void *start, size_t len_in, const char *name)
{
+ if (TUNABLE_GET (glibc, mem, decorate_maps, int32_t, NULL) == 0)
+ return;
+
static int prctl_supported = 1;
if (atomic_load_relaxed (&prctl_supported) == 0)
return;
The PR_SET_VMA_ANON_NAME support is only enabled through a configurable kernel switch, mainly because assigning a name to a anonymous virtual memory area might prevent that area from being merged with adjacent virtual memory areas. For instance, with the following code: void *p1 = mmap (NULL, 1024 * 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); void *p2 = mmap (p1 + (1024 * 4096), 1024 * 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); The kernel will potentially merge both mappings resulting in only one segment of size 0x800000. If the segment is names with PR_SET_VMA_ANON_NAME with different names, it results in two mappings. Although this will unlikely be an issue for pthread stacks and malloc arenas (since for pthread stacks the guard page will result in a PROT_NONE segment, similar to the alignment requirement for the arena block), it still might prevent the mmap memory allocated for detail malloc. There is also another potential scalability issue, where the prctl requires to take the mmap global lock which is still not fully fixed in Linux [1] (for pthread stacks and arenas, it is mitigated by the stack cached and the arena reuse). So this patch disables anonymous mapping annotations as default and add a new tunable, glibc.mem.decorate_maps, can be used to enable it. [1] https://lwn.net/Articles/906852/ Checked on x86_64-linux-gnu and aarch64-linux-gnu. --- NEWS | 5 +++++ elf/Makefile | 2 +- elf/dl-tunables.list | 5 +++++ manual/tunables.texi | 12 ++++++++++++ sysdeps/unix/sysv/linux/setvmaname.c | 4 ++++ 5 files changed, 27 insertions(+), 1 deletion(-)