[PATCH] cmsg.3, getaddrinfo_a.3 getgrouplist.3: Use sizeof, consistently

Alejandro Colomar
2020-08-25T13:05:49Z

Hi Michael,

On 8/25/20 2:35 PM, Michael Kerrisk (man-pages) wrote:
> Patch applied, but I had to fix up the title line, which got taken
> from the mail subject line. Probably best to send subsequent patches
> as new mails, rather than as replies to this thread.

This time I replied to the thread (so that the conversation can be
followed in the archive), but changed the subject. I hope that works.

(I sent this email only to you by accident; I'm sending it again with
the CCs).

Patch (3) as numbered in the previous replies:

--------------------------------------------------------------------
From 1cb973629d94a048c5dcbe13fef76173f99dc3de Mon Sep 17 00:00:00 2001
From: Alejandro Colomar <colomar.6.4.3@gmail.com>
Date: Tue, 25 Aug 2020 14:52:03 +0200
Subject: [PATCH] cmsg.3, getaddrinfo_a.3 getgrouplist.3: Use sizeof
consistently

Use ``sizeof`` consistently through all the examples in the following
way:

- When the result of ``sizeof`` is multiplied (or otherwise modified),
write ``sizeof`` in the first place.

Rationale:

``(sizeof(x) * INT_MAX * 2)`` doesn't overflow.

``(INT_MAX * 2 * sizeof(x))`` overflows, giving incorrect
results.

As a side effect, the parentheses of ``sizeof`` are not next to
the parentheses of the whole expression, and it is visually
easier to read.

Detailed rationale:

In C, successive multiplications are evaluated left to right (*), and
therefore here is what happens (assuming x86_64):

``(sizeof(x) * INT_MAX * 2)``:

1) sizeof(x) * INT_MAX (the type is the largest of both, which is
size_t (unsigned long; uint64_t)).
2) ANS * 2 (the type is again the largest: size_t)

``(INT_MAX * 2 * sizeof(x))``:

1) INT_MAX * 2 (the type is the largest of both, which is
int as both are int (int; int32_t), so the
result is already truncated as it doesn't fit
an int; at this point, the intermediate result
will be 2^32 - 2 (``INT_MAX - 1``) (if I did
the math right)).
2) ANS * 2 (the type is again the largest of both: size_t;
however, ANS was already incorrect, so the
result will be an incorrect size_t value)

(*): https://en.cppreference.com/w/c/language/operator_precedence

Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
---
man3/cmsg.3 | 2 +-
man3/getaddrinfo_a.3 | 2 +-
man3/getgrouplist.3 | 2 +-
3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/man3/cmsg.3 b/man3/cmsg.3
index 2f9910093..99ee950f9 100644
--- a/man3/cmsg.3
+++ b/man3/cmsg.3
@@ -242,7 +242,7 @@ cmsg = CMSG_FIRSTHDR(&msg);
cmsg\->cmsg_level = SOL_SOCKET;
cmsg\->cmsg_type = SCM_RIGHTS;
cmsg\->cmsg_len = CMSG_LEN(sizeof(int) * NUM_FD);
-memcpy(CMSG_DATA(cmsg), myfds, NUM_FD * sizeof(int));
+memcpy(CMSG_DATA(cmsg), myfds, sizeof(int) * NUM_FD);
.EE
.in
.SH SEE ALSO
diff --git a/man3/getaddrinfo_a.3 b/man3/getaddrinfo_a.3
index af8f88937..cd4cad0dc 100644
--- a/man3/getaddrinfo_a.3
+++ b/man3/getaddrinfo_a.3
@@ -473,7 +473,7 @@ add_requests(void)

while ((host = strtok(NULL, " "))) {
nreqs++;
- reqs = realloc(reqs, nreqs * sizeof(reqs[0]));
+ reqs = realloc(reqs, sizeof(reqs[0]) * nreqs);

reqs[nreqs \- 1] = calloc(1, sizeof(*reqs[0]));
reqs[nreqs \- 1]\->ar_name = strdup(host);
diff --git a/man3/getgrouplist.3 b/man3/getgrouplist.3
index aea52d999..372f2613f 100644
--- a/man3/getgrouplist.3
+++ b/man3/getgrouplist.3
@@ -164,7 +164,7 @@ main(int argc, char *argv[])

ngroups = atoi(argv[2]);

- groups = malloc(ngroups * sizeof(gid_t));
+ groups = malloc(sizeof(gid_t) * ngroups);
if (groups == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
--
2.28.0