exec: Make ARG_MAX accounting a bit less restrictive with large pages

For Linux-style accounting, we really only need to handle a single page
of wasted space due to rounding.  Subtracting two pages for extra
headroom was reasonable on systems with 4K pages, but overkill on
systems like ppc64le with 64K pages.  Worse yet was the fact that Alpine
Linux only gives us 128K for arguments.

Instead, only subtract a single page, plus the POSIX-recommended 2048
bytes.

Credit to Mike Sullivan for the initial patch and testing on Alpine
ppc64le.

Fixes:
http://build.alpinelinux.org/buildlogs/build-edge-ppc64le/testing/bfs/bfs-1.2.2-r0.log

Co-authored-by: Mike Sullivan <mksully22@gmail.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/exec.c b/exec.c
index 2b18ce8..d383935 100644
--- a/exec.c
+++ b/exec.c
@@ -90,13 +90,16 @@ static size_t bfs_exec_arg_max(const struct bfs_exec *execbuf) {
 	bfs_exec_debug(execbuf, "ARG_MAX: %ld remaining after fixed arguments\n", arg_max);
 
 	// 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
+	// so allow a one page cushion to account for rounding up
 	long page_size = sysconf(_SC_PAGESIZE);
 	if (page_size < 4096) {
 		page_size = 4096;
 	}
-	arg_max -= 2*page_size;
+	arg_max -= page_size;
+	bfs_exec_debug(execbuf, "ARG_MAX: %ld remaining after page cushion\n", arg_max);
+
+	// POSIX recommends an additional 2048 bytes of headroom
+	arg_max -= 2048;
 	bfs_exec_debug(execbuf, "ARG_MAX: %ld remaining after headroom\n", arg_max);
 
 	if (arg_max < 0) {