[PATCH 3/4] Implement support for checking /proc/PID/coredump_filter

Sergio Durigan Junior
2015-03-18T19:39:29Z

This is the core patch. It implements the support for honoring the
/proc/PID/coredump_filter file.

The code haven't changed much from v2. I basically included more
comments (or extended the existing ones) explaining things that Pedro
asked.

Changes from v2:

- Improved error mechanism on mapping_is_anonymous_p.

- Improved comments on mapping_is_anonymous_p.

gdb/ChangeLog:
2015-03-18 Sergio Durigan Junior <sergiodj@redhat.com>
Jan Kratochvil <jan.kratochvil@redhat.com>
Oleg Nesterov <oleg@redhat.com>

PR corefiles/16092
* linux-tdep.c: Include 'gdbcmd.h' and 'gdb_regex.h'.
New enum identifying the various options of the coredump_filter
file.
(struct smaps_vmflags): New struct.
(use_coredump_filter): New variable.
(decode_vmflags): New function.
(mapping_is_anonymous_p): Likewise.
(dump_mapping_p): Likewise.
(linux_find_memory_regions_full): New variables
'coredumpfilter_name', 'coredumpfilterdata', 'pid',
'filterflags'. Read /proc/<PID>/smaps file; improve parsing of
its information. Implement memory mapping filtering based on its
contents.
(show_use_coredump_filter): New function.
(_initialize_linux_tdep): New command 'set use-coredump-filter'.
---
gdb/linux-tdep.c | 403 +++++++++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 379 insertions(+), 24 deletions(-)

diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index 15725c7..345c178 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -35,9 +35,61 @@
#include "observer.h"
#include "objfiles.h"
#include "infcall.h"
+#include "gdbcmd.h"
+#include "gdb_regex.h"

#include <ctype.h>

+/* This enum represents the values that the user can choose when
+ informing the Linux kernel about which memory mappings will be
+ dumped in a corefile. They are described in the file
+ Documentation/filesystems/proc.txt, inside the Linux kernel
+ tree. */
+
+enum
+ {
+ COREFILTER_ANON_PRIVATE = 1 << 0,
+ COREFILTER_ANON_SHARED = 1 << 1,
+ COREFILTER_MAPPED_PRIVATE = 1 << 2,
+ COREFILTER_MAPPED_SHARED = 1 << 3,
+ COREFILTER_ELF_HEADERS = 1 << 4,
+ COREFILTER_HUGETLB_PRIVATE = 1 << 5,
+ COREFILTER_HUGETLB_SHARED = 1 << 6,
+ };
+
+/* This struct is used to map flags found in the "VmFlags:" field (in
+ the /proc/<PID>/smaps file). */
+
+struct smaps_vmflags
+ {
+ /* Zero if this structure has not been initialized yet. It
+ probably means that the Linux kernel being used does not emit
+ the "VmFlags:" field on "/proc/PID/smaps". */
+
+ unsigned int initialized_p : 1;
+
+ /* Memory mapped I/O area (VM_IO, "io"). */
+
+ unsigned int io_page : 1;
+
+ /* Area uses huge TLB pages (VM_HUGETLB, "ht"). */
+
+ unsigned int uses_huge_tlb : 1;
+
+ /* Do not include this memory region on the coredump (VM_DONTDUMP, "dd"). */
+
+ unsigned int exclude_coredump : 1;
+
+ /* Is this a MAP_SHARED mapping (VM_SHARED, "sh"). */
+
+ unsigned int shared_mapping : 1;
+ };
+
+/* Whether to take the /proc/PID/coredump_filter into account when
+ generating a corefile. */
+
+static int use_coredump_filter = 1;
+
/* This enum represents the signals' numbers on a generic architecture
running the Linux kernel. The definition of "generic" comes from
the file <include/uapi/asm-generic/signal.h>, from the Linux kernel
@@ -381,6 +433,220 @@ read_mapping (const char *line,
*filename = p;
}

+/* Helper function to decode the "VmFlags" field in /proc/PID/smaps.
+
+ This function was based on the documentation found on
+ <Documentation/filesystems/proc.txt>, on the Linux kernel.
+
+ Linux kernels before commit
+ 834f82e2aa9a8ede94b17b656329f850c1471514 do not have this field on
+ smaps. */
+
+static void
+decode_vmflags (char *p, struct smaps_vmflags *v)
+{
+ char *saveptr;
+ const char *s;
+
+ v->initialized_p = 1;
+ p = skip_to_space (p);
+ p = skip_spaces (p);
+
+ for (s = strtok_r (p, " ", &saveptr);
+ s != NULL;
+ s = strtok_r (NULL, " ", &saveptr))
+ {
+ if (strcmp (s, "io") == 0)
+ v->io_page = 1;
+ else if (strcmp (s, "ht") == 0)
+ v->uses_huge_tlb = 1;
+ else if (strcmp (s, "dd") == 0)
+ v->exclude_coredump = 1;
+ else if (strcmp (s, "sh") == 0)
+ v->shared_mapping = 1;
+ }
+}
+
+/* Return 1 if the memory mapping is anonymous, 0 otherwise.
+
+ FILENAME is the name of the file present in the first line of the
+ memory mapping, in the "/proc/PID/smaps" output. For example, if
+ the first line is:
+
+ 7fd0ca877000-7fd0d0da0000 r--p 00000000 fd:02 2100770 /path/to/file
+
+ Then FILENAME will be "/path/to/file". */
+
+static int
+mapping_is_anonymous_p (const char *filename)
+{
+ static regex_t dev_zero_regex, shmem_file_regex, file_deleted_regex;
+ static int init_regex_p = 0;
+
+ if (!init_regex_p)
+ {
+ struct cleanup *c = make_cleanup (null_cleanup, NULL);
+
+ /* Let's be pessimistic and assume there will be an error while
+ compiling the regex'es. */
+ init_regex_p = -1;
+
+ /* DEV_ZERO_REGEX matches "/dev/zero" filenames (with or
+ without the "(deleted)" string in the end). We know for
+ sure, based on the Linux kernel code, that memory mappings
+ whose associated filename is "/dev/zero" are guaranteed to be
+ MAP_ANONYMOUS. */
+ compile_rx_or_error (&dev_zero_regex, "^/dev/zero\\( (deleted)\\)\\?$",
+ _("Could not compile regex to match /dev/zero "
+ "filename"));
+ /* SHMEM_FILE_REGEX matches "/SYSV%08x" filenames (with or
+ without the "(deleted)" string in the end). These filenames
+ refer to shared memory (shmem), and memory mappings
+ associated with them are MAP_ANONYMOUS as well. */
+ compile_rx_or_error (&shmem_file_regex,
+ "^/\\?SYSV[0-9a-fA-F]\\{8\\}\\( (deleted)\\)\\?$",
+ _("Could not compile regex to match shmem "
+ "filenames"));
+ /* FILE_DELETED_REGEX is a heuristic we use to try to mimic the
+ Linux kernel's 'n_link == 0' code, which is responsible to
+ decide if it is dealing with a 'MAP_SHARED | MAP_ANONYMOUS'
+ mapping. In other words, if FILE_DELETED_REGEX matches, it
+ does not necessarily mean that we are dealing with an
+ anonymous shared mapping. However, there is no easy way to
+ detect this currently, so this is the best approximation we
+ have.
+
+ As a result, GDB will dump readonly pages of deleted
+ executables when using the default value of coredump_filter
+ (0x33), while the Linux kernel will not dump those pages.
+ But we can live with that. */
+ compile_rx_or_error (&file_deleted_regex, " (deleted)$",
+ _("Could not compile regex to match "
+ "'<file> (deleted)'"));
+ /* We will never release these regexes, so just discard the
+ cleanups. */
+ discard_cleanups (c);
+
+ /* If we reached this point, then everything succeeded. */
+ init_regex_p = 1;
+ }
+
+ if (init_regex_p == -1)
+ {
+ /* There was an error while compiling the regex'es above. In
+ order to try to give some reliable information to the caller,
+ we just try to find the string "(deleted)" in the filename.
+ If we managed to find it, then we assume the mapping is
+ anonymous. */
+ return strstr (filename, "(deleted)") != NULL;
+ }
+
+ if (*filename == '\0'
+ || regexec (&dev_zero_regex, filename, 0, NULL, 0) == 0
+ || regexec (&shmem_file_regex, filename, 0, NULL, 0) == 0
+ || regexec (&file_deleted_regex, filename, 0, NULL, 0) == 0)
+ return 1;
+
+ return 0;
+}
+
+/* Return 0 if the memory mapping (which is related to FILTERFLAGS, V,
+ MAYBE_PRIVATE_P, and MAPPING_ANONYMOUS_P) should not be dumped, or
+ greater than 0 if it should.
+
+ In a nutshell, this is the logic that we follow in order to decide
+ if a mapping should be dumped or not.
+
+ - If the mapping is associated to a file whose name ends with "
+ (deleted)", or if the file is "/dev/zero", or if it is
+ "/SYSV%08x" (shared memory), or if there is no file associated
+ with it, or if the AnonHugePages: or the Anonymous: fields in the
+ /proc/PID/smaps have contents, then GDB considers this mapping to
+ be anonymous. Otherwise, GDB considers this mapping to be a
+ file-backed mapping (because there will be a file associated with
+ it).
+
+ It is worth mentioning that, from all those checks described
+ above, the most fragile is the one to see if the file name ends
+ with " (deleted)". This does not necessarily mean that the
+ mapping is anonymous, because the deleted file associated with
+ the mapping may have been a hard link to another file, for
+ example. The Linux kernel checks to see if "i_nlink == 0", but
+ GDB cannot easily do this check. Therefore, we made a compromise
+ here, and we assume that if the file name ends with " (deleted)",
+ then the mapping is indeed anonymous. FWIW, this is something
+ the Linux kernel could do better: expose this information in a
+ more direct way.
+
+ - If we see the flag "sh" in the "VmFlags:" field (in
+ /proc/PID/smaps), then certainly the memory mapping is shared
+ (VM_SHARED). If we have access to the VmFlags, and we don't see
+ the "sh" there, then certainly the mapping is private. However,
+ Linux kernels before commit
+ 834f82e2aa9a8ede94b17b656329f850c1471514 do not have the
+ "VmFlags:" field; in that case, we use another heuristic: if we
+ see 'p' in the permission flags, then we assume that the mapping
+ is private, even though the presence of the 's' flag there would
+ mean VM_MAYSHARE, which means the mapping could still be private.
+ This should work OK enough, however. */
+
+static int
+dump_mapping_p (unsigned int filterflags, const struct smaps_vmflags *v,
+ int maybe_private_p, int mapping_anon_p, const char *filename)
+{
+ /* Initially, we trust in what we received from outside. This value
+ may not be very precise (i.e., it was probably gathered from the
+ permission line in the /proc/PID/smaps list, which actually
+ refers to VM_MAYSHARE, and not VM_SHARED), but it is what we have
+ for now. */
+ int private_p = maybe_private_p;
+
+ /* We always dump vDSO and vsyscall mappings. */
+ if (strcmp ("[vdso]", filename) == 0
+ || strcmp ("[vsyscall]", filename) == 0)
+ return 1;
+
+ if (v->initialized_p)
+ {
+ /* We never dump I/O mappings. */
+ if (v->io_page)
+ return 0;
+
+ /* Check if we should exclude this mapping. */
+ if (v->exclude_coredump)
+ return 0;
+
+ /* Update our notion of whether this mapping is shared or
+ private based on a trustworthy value. */
+ private_p = !v->shared_mapping;
+
+ /* HugeTLB checking. */
+ if (v->uses_huge_tlb)
+ {
+ if ((private_p && (filterflags & COREFILTER_HUGETLB_PRIVATE))
+ || (!private_p && (filterflags & COREFILTER_HUGETLB_SHARED)))
+ return 1;
+
+ return 0;
+ }
+ }
+
+ if (private_p)
+ {
+ if (mapping_anon_p)
+ return (filterflags & COREFILTER_ANON_PRIVATE) != 0;
+ else
+ return (filterflags & COREFILTER_MAPPED_PRIVATE) != 0;
+ }
+ else
+ {
+ if (mapping_anon_p)
+ return (filterflags & COREFILTER_ANON_SHARED) != 0;
+ else
+ return (filterflags & COREFILTER_MAPPED_SHARED) != 0;
+ }
+}
+
/* Implement the "info proc" command. */

static void
@@ -821,49 +1087,85 @@ linux_find_memory_regions_full (struct gdbarch *gdbarch,
void *obfd)
{
char mapsfilename[100];
- char *data;
+ char coredumpfilter_name[100];
+ char *data, *coredumpfilterdata;
+ pid_t pid;
+ /* Default dump behavior of coredump_filter (0x33), according to
+ Documentation/filesystems/proc.txt from the Linux kernel
+ tree. */
+ unsigned int filterflags = (COREFILTER_ANON_PRIVATE
+ | COREFILTER_ANON_SHARED
+ | COREFILTER_ELF_HEADERS
+ | COREFILTER_HUGETLB_PRIVATE);

/* We need to know the real target PID to access /proc. */
if (current_inferior ()->fake_pid_p)
return 1;

- xsnprintf (mapsfilename, sizeof mapsfilename,
- "/proc/%d/smaps", current_inferior ()->pid);
+ pid = current_inferior ()->pid;
+
+ if (use_coredump_filter)
+ {
+ xsnprintf (coredumpfilter_name, sizeof (coredumpfilter_name),
+ "/proc/%d/coredump_filter", pid);
+ coredumpfilterdata = target_fileio_read_stralloc (coredumpfilter_name);
+ if (coredumpfilterdata != NULL)
+ {
+ sscanf (coredumpfilterdata, "%x", &filterflags);
+ xfree (coredumpfilterdata);
+ }
+ }
+
+ xsnprintf (mapsfilename, sizeof mapsfilename, "/proc/%d/smaps", pid);
data = target_fileio_read_stralloc (mapsfilename);
if (data == NULL)
{
/* Older Linux kernels did not support /proc/PID/smaps. */
- xsnprintf (mapsfilename, sizeof mapsfilename,
- "/proc/%d/maps", current_inferior ()->pid);
+ xsnprintf (mapsfilename, sizeof mapsfilename, "/proc/%d/maps", pid);
data = target_fileio_read_stralloc (mapsfilename);
}
- if (data)
+
+ if (data != NULL)
{
struct cleanup *cleanup = make_cleanup (xfree, data);
- char *line;
+ char *line, *t;

- line = strtok (data, "\n");
- while (line)
+ line = strtok_r (data, "\n", &t);
+ while (line != NULL)
{
ULONGEST addr, endaddr, offset, inode;
const char *permissions, *device, *filename;
+ struct smaps_vmflags v;
size_t permissions_len, device_len;
- int read, write, exec;
- enum memory_mapping_state modified_state = MEMORY_MAPPING_UNMODIFIED;
+ int read, write, exec, private;
+ enum memory_mapping_state modified_state;
int has_anonymous = 0;
+ int mapping_anon_p;

+ memset (&v, 0, sizeof (v));
read_mapping (line, &addr, &endaddr, &permissions, &permissions_len,
&offset, &device, &device_len, &inode, &filename);
+ mapping_anon_p = mapping_is_anonymous_p (filename);

/* Decode permissions. */
read = (memchr (permissions, 'r', permissions_len) != 0);
write = (memchr (permissions, 'w', permissions_len) != 0);
exec = (memchr (permissions, 'x', permissions_len) != 0);
+ /* 'private' here actually means VM_MAYSHARE, and not
+ VM_SHARED. In order to know if a mapping is really
+ private or not, we must check the flag "sh" in the
+ VmFlags field. This is done by decode_vmflags. However,
+ if we are using a Linux kernel released before the commit
+ 834f82e2aa9a8ede94b17b656329f850c1471514, we will not
+ have the VmFlags there. In this case, there is really no
+ way to know if we are dealing with VM_SHARED, so we just
+ assume that VM_MAYSHARE is enough. */
+ private = memchr (permissions, 'p', permissions_len) != 0;

/* Try to detect if region was modified by parsing smaps counters. */
- for (line = strtok (NULL, "\n");
- line && line[0] >= 'A' && line[0] <= 'Z';
- line = strtok (NULL, "\n"))
+ for (line = strtok_r (NULL, "\n", &t);
+ line != NULL && line[0] >= 'A' && line[0] <= 'Z';
+ line = strtok_r (NULL, "\n", &t))
{
char keyword[64 + 1];

@@ -872,11 +1174,17 @@ linux_find_memory_regions_full (struct gdbarch *gdbarch,
warning (_("Error parsing {s,}maps file '%s'"), mapsfilename);
break;
}
+
if (strcmp (keyword, "Anonymous:") == 0)
- has_anonymous = 1;
- if (strcmp (keyword, "Shared_Dirty:") == 0
- || strcmp (keyword, "Private_Dirty:") == 0
- || strcmp (keyword, "Swap:") == 0
+ {
+ /* Older Linux kernels did not support the
+ "Anonymous:" counter. Check it here. */
+ has_anonymous = 1;
+ }
+ else if (strcmp (keyword, "VmFlags:") == 0)
+ decode_vmflags (line, &v);
+
+ if (strcmp (keyword, "AnonHugePages:") == 0
|| strcmp (keyword, "Anonymous:") == 0)
{
unsigned long number;
@@ -887,15 +1195,39 @@ linux_find_memory_regions_full (struct gdbarch *gdbarch,
mapsfilename);
break;
}
- if (number != 0)
- modified_state = MEMORY_MAPPING_MODIFIED;
+ if (number > 0)
+ {
+ /* Even if we are dealing with a file-backed
+ mapping, if it contains anonymous pages we
+ consider it to be an anonymous mapping,
+ because this is what the Linux kernel does:
+
+ // Dump segments that have been written to.
+ if (vma->anon_vma && FILTER(ANON_PRIVATE))
+ goto whole;
+ */
+ mapping_anon_p = 1;
+ }
}
}

- /* Older Linux kernels did not support the "Anonymous:" counter.
- If it is missing, we can't be sure - dump all the pages. */
- if (!has_anonymous)
- modified_state = MEMORY_MAPPING_MODIFIED;
+ /* If a mapping should not be dumped we still should create
+ a segment for it, just without SEC_LOAD (see
+ gcore_create_callback). */
+ if (has_anonymous)
+ {
+ if (dump_mapping_p (filterflags, &v, private, mapping_anon_p,
+ filename))
+ modified_state = MEMORY_MAPPING_MODIFIED;
+ else
+ modified_state = MEMORY_MAPPING_UNMODIFIED;
+ }
+ else
+ {
+ /* Older Linux kernels did not support the "Anonymous:" counter.
+ If it is missing, we can't be sure - dump all the pages. */
+ modified_state = MEMORY_MAPPING_UNKNOWN_STATE;
+ }

/* Invoke the callback function to create the corefile segment. */
func (addr, endaddr - addr, offset, inode,
@@ -1979,6 +2311,17 @@ linux_infcall_mmap (CORE_ADDR size, unsigned prot)
return retval;
}

+/* Display whether the gcore command is using the
+ /proc/PID/coredump_filter file. */
+
+static void
+show_use_coredump_filter (struct ui_file *file, int from_tty,
+ struct cmd_list_element *c, const char *value)
+{
+ fprintf_filtered (file, _("Use of /proc/PID/coredump_filter file to generate"
+ " corefiles is %s.\n"), value);
+}
+
/* To be called from the various GDB_OSABI_LINUX handlers for the
various GNU/Linux architectures and machine types. */

@@ -2015,4 +2358,16 @@ _initialize_linux_tdep (void)
/* Observers used to invalidate the cache when needed. */
observer_attach_inferior_exit (invalidate_linux_cache_inf);
observer_attach_inferior_appeared (invalidate_linux_cache_inf);
+
+ add_setshow_boolean_cmd ("use-coredump-filter", class_files,
+ &use_coredump_filter, _("\
+Set whether gcore should consider /proc/PID/coredump_filter."),
+ _("\
+Show whether gcore should consider /proc/PID/coredump_filter."),
+ _("\
+Use this command to set whether gcore should consider the contents\n\
+of /proc/PID/coredump_filter when generating the corefile. For more information\n\
+about this file, refer to the manpage of core(5)."),
+ NULL, show_use_coredump_filter,
+ &setlist, &showlist);
}
--
1.9.3