From: Tom Lane Date: Tue, 1 Jul 2025 16:08:20 +0000 (-0400) Subject: Make safeguard against incorrect flags for fsync more portable. X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=71d71ac4d1bbb34a9814f010812e059c29c9472e;p=thirdparty%2Fpostgresql.git Make safeguard against incorrect flags for fsync more portable. The existing code assumed that O_RDONLY is defined as 0, but this is not required by POSIX and is not true on GNU Hurd. We can avoid the assumption by relying on O_ACCMODE to mask the fcntl() result. (Hopefully, all supported platforms define that.) Author: Michael Banck Co-authored-by: Samuel Thibault Reviewed-by: Tom Lane Discussion: https://postgr.es/m/6862e8d1.050a0220.194b8d.76fa@mx.google.com Discussion: https://postgr.es/m/68480868.5d0a0220.1e214d.68a6@mx.google.com Backpatch-through: 13 --- diff --git a/src/backend/storage/file/fd.c b/src/backend/storage/file/fd.c index 484a07302a4..e74248ae4f9 100644 --- a/src/backend/storage/file/fd.c +++ b/src/backend/storage/file/fd.c @@ -366,25 +366,22 @@ pg_fsync(int fd) * portable, even if it runs ok on the current system. * * We assert here that a descriptor for a file was opened with write - * permissions (either O_RDWR or O_WRONLY) and for a directory without - * write permissions (O_RDONLY). + * permissions (i.e., not O_RDONLY) and for a directory without write + * permissions (O_RDONLY). Notice that the assertion check is made even + * if fsync() is disabled. * - * Ignore any fstat errors and let the follow-up fsync() do its work. - * Doing this sanity check here counts for the case where fsync() is - * disabled. + * If fstat() fails, ignore it and let the follow-up fsync() complain. */ if (fstat(fd, &st) == 0) { int desc_flags = fcntl(fd, F_GETFL); - /* - * O_RDONLY is historically 0, so just make sure that for directories - * no write flags are used. - */ + desc_flags &= O_ACCMODE; + if (S_ISDIR(st.st_mode)) - Assert((desc_flags & (O_RDWR | O_WRONLY)) == 0); + Assert(desc_flags == O_RDONLY); else - Assert((desc_flags & (O_RDWR | O_WRONLY)) != 0); + Assert(desc_flags != O_RDONLY); } errno = 0; #endif