[PATCH] procfs: Add mem_end to /proc/<pid>/stat

[Christopher Covington]

Applications such as Just-In-Time (JIT) compilers, Checkpoint/Restore In
Userspace (CRIU), and User Mode Linux (UML) need to know the highest
virtual address, TASK_SIZE, to implement pointer tagging or make a first
educated guess at where to find a large, unused region of memory.
Unfortunately the currently available mechanisms for determining TASK_SIZE
are either convoluted and potentially error-prone, such as making repeated
munmap() calls and checking the return code, or make use of hard-coded
assumptions that limit an application's portability across kernels with
different Kconfig options and multiple architectures.

Therefore, expose TASK_SIZE to userspace. While PAGE_SIZE is exposed to
userspace via an auxiliary vector, that approach is not used for TASK_SIZE
in case run-time alterations to the usable virtual address range are one
day implemented, such as through an extension to prctl(PR_SET_MM) or a flag
to clone. There is no prctl(PR_GET_MM). Instead such information is
expected to come from /proc/<pid>/stat[m]. For the same extendability
reason, use a per-pid proc entry rather than a system-wide entry like
/proc/sys/vm/mmap_min_addr.

Signed-off-by: Christopher Covington <cov@xxxxxxxxxxxxx>
---
 Documentation/filesystems/proc.txt | 1 +
 fs/proc/array.c                    | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 74329fd..b9c19cf 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -343,6 +343,7 @@ Table 1-4: Contents of the stat files (as of 2.6.30-rc7)
   env_start     address above which program environment is placed
   env_end       address below which program environment is placed
   exit_code     the thread's exit_code in the form reported by the waitpid system call
+  end_mem       address below which all regular program parts are placed (TASK_SIZE)
 ..............................................................................
 
 The /proc/PID/maps file containing the currently mapped memory regions and
diff --git a/fs/proc/array.c b/fs/proc/array.c
index 9a3ca9e..32b5002 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -561,6 +561,11 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 	else
 		seq_puts(m, " 0");
 
+	if (mm && permitted)
+		seq_put_decimal_ull(m, " ", mm->task_size);
+	else
+		seq_puts(m, " 0");
+
 	seq_putc(m, '\n');
 	if (mm)
 		mmput(mm);
-- 
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