exec: Make argument size tracking robust to page-granularity accounting

From looking at the Linux exec() implementation, it seems a big part of
the reason we needed extra headroom was that the arguments/environment
are copied page-by-page, so even a small accounting difference could
result in an error of an entire page size.  Grow the headroom to two
entire pages to account for this.

1 files changed, 8 insertions, 3 deletions

diff --git a/exec.c b/exec.c
index b982f85..23ea0c7 100644
--- a/exec.c
+++ b/exec.c
@@ -89,9 +89,14 @@ static size_t bfs_exec_arg_max(const struct bfs_exec *execbuf) {
 	arg_max -= sizeof(char *);
 	bfs_exec_debug(execbuf, "ARG_MAX: %ld remaining after fixed arguments\n", arg_max);
 
-	// POSIX recommends subtracting 2048, for some wiggle room
-	// We subtract 4096 for extra insurance, based on some experimentation
-	arg_max -= 4096;
+	// Assume arguments are counted with the granularity of a single page,
+	// and allow two pages of headroom to account for rounding as well as
+	// any other data we may not be counting
+	long page_size = sysconf(_SC_PAGESIZE);
+	if (page_size < 4096) {
+		page_size = 4096;
+	}
+	arg_max -= 2*page_size;
 	bfs_exec_debug(execbuf, "ARG_MAX: %ld remaining after headroom\n", arg_max);
 
 	if (arg_max < 0) {