[PATCH] stdio: fix scanf bug

[Nadav Har'El]

Lewis Tian noted on the mailing list that an application reading with
fscanf() from a large file fails after passing one BUFSIZ of data.

It turns out that the old Musl code we were using was only correct assuming
a certain bizarre readv() bug workaround which OSv dropped in commit
0b651428b91663255d8da9a4913663d0cd4cc710. But the Musl developers have
too noticed they can't rely on this workaround (which doesn't even apply
to unbuffered streams), and came up with a fix in their commit

https://git.musl-libc.org/cgit/musl/commit/src/internal/shgetc.c?id=c20804500deebaabc56f383d48dd1ac77dce8349

This patch does the same fix in our copy of the Musl code.
By the way, after this change libc/internal/shgetc.c is now identical to
musl/src/internal/shgetc.c but we can't drop the former yet because of some
header file differences that still need to be cleared up.

Here is the original Musl commit message describing the change:

"fix major scanf breakage with unbuffered streams, fmemopen, etc.
the shgetc api, used internally in scanf and int/float scanning code
to handle field width limiting and pushback, was designed assuming
that pushback could be achieved via a simple decrement on the file
buffer pointer. this only worked by chance for regular FILE streams,
due to the linux readv bug workaround in __stdio_read which moves the
last requested byte through the buffer rather than directly back to
the caller. for unbuffered streams and streams not using __stdio_read
but some other underlying read function, the first character read
could be completely lost, and replaced by whatever junk happened to be
in the unget buffer.

to fix this, simply have shgetc, when it performs an underlying read
operation on the stream, store the character read at the -1 offset
from the read buffer pointer. this is valid even for unbuffered
streams, as they have an unget buffer located just below the start of
the zero-length buffer. the check to avoid storing the character when
it is already there is to handle the possibility of read-only buffers.
no application-exposed FILE types are allowed to use read-only
buffers, but sscanf and strto* may use them internally when calling
functions which use the shgetc api."

Signed-off-by: Nadav Har'El <n...@scylladb.com>
---
libc/internal/shgetc.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/libc/internal/shgetc.c b/libc/internal/shgetc.c
index 0641c835..5a47ac9e 100644
--- a/libc/internal/shgetc.c
+++ b/libc/internal/shgetc.c
@@ -22,5 +22,6 @@ int __shgetc(FILE *f)
else
f->shend = f->rend;
if (f->rend) f->shcnt += f->rend - f->rpos + 1;
+ if (f->rpos[-1] != c) f->rpos[-1] = c;
return c;
}
--
2.21.1

[Commit Bot]

From: Nadav Har'El <n...@scylladb.com>
Committer: Waldemar Kozaczuk <jwkoz...@gmail.com>
Branch: master

stdio: fix scanf bug

Message-Id: <2020011411183...@scylladb.com>

---
diff --git a/libc/internal/shgetc.c b/libc/internal/shgetc.c