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copy: rework how we determine the number of bytes to copy in copy_bytes_full()

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Let's freshly calculate "m" on each iteration and always start with the maximum
size we can. If sendfile() is used we must adhere to its limit of
SSIZE_MAX minus the current offset. Otherwise we can copy more, i.e.
SSIZE_MAX without any restrictions.

Also, if we get too close to having copied SSIZE_MAX, let's turn off
sendfile() for the rest.
This commit is contained in:
Lennart Poettering 2024-05-24 12:02:42 +02:00
parent 432977a0a4
commit 22ed8700c7
1 changed files with 20 additions and 12 deletions
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32
src/shared/copy.c
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@ -162,9 +162,9 @@ int copy_bytes_full(
void *userdata) {
_cleanup_close_ int fdf_opened = -EBADF, fdt_opened = -EBADF;
bool try_cfr = true, try_sendfile = true, try_splice = true, copied_something = false;
bool try_cfr = true, try_sendfile = true, try_splice = true;
uint64_t copied_total = 0;
int r, nonblock_pipe = -1;
size_t m = SSIZE_MAX; /* that is the maximum that sendfile and c_f_r accept */
assert(fdf >= 0);
assert(fdt >= 0);
@ -264,6 +264,7 @@ int copy_bytes_full(
for (;;) {
ssize_t n;
size_t m;
if (max_bytes <= 0)
break;
@ -272,6 +273,14 @@ int copy_bytes_full(
if (r < 0)
return r;
/* sendfile() accepts at most SSIZE_MAX-offset bytes to copy, hence let's subtract how much
* copied so far from SSIZE_MAX as maximum of what we want to copy. */
if (try_sendfile) {
assert(copied_total < SSIZE_MAX);
m = (uint64_t) SSIZE_MAX - copied_total;
} else
m = SSIZE_MAX;
if (max_bytes != UINT64_MAX && m > max_bytes)
m = max_bytes;
@ -342,7 +351,7 @@ int copy_bytes_full(
/* use fallback below */
} else if (n == 0) { /* likely EOF */
if (copied_something)
if (copied_total > 0)
break;
/* So, we hit EOF immediately, without having copied a single byte. This
@ -369,7 +378,7 @@ int copy_bytes_full(
/* use fallback below */
} else if (n == 0) { /* likely EOF */
if (copied_something)
if (copied_total > 0)
break;
try_sendfile = try_splice = false; /* same logic as above for copy_file_range() */
@ -432,7 +441,7 @@ int copy_bytes_full(
/* use fallback below */
} else if (n == 0) { /* likely EOF */
if (copied_something)
if (copied_total > 0)
break;
try_splice = false; /* same logic as above for copy_file_range() + sendfile() */
@ -483,6 +492,12 @@ int copy_bytes_full(
}
next:
copied_total += n;
/* Disable sendfile() in case we are getting too close to it's SSIZE_MAX-offset limit */
if (copied_total > SSIZE_MAX - COPY_BUFFER_SIZE)
try_sendfile = false;
if (progress) {
r = progress(n, userdata);
if (r < 0)
@ -493,13 +508,6 @@ int copy_bytes_full(
assert(max_bytes >= (uint64_t) n);
max_bytes -= n;
}
/* sendfile accepts at most SSIZE_MAX-offset bytes to copy, so reduce our maximum by the
* amount we already copied, but don't go below our copy buffer size, unless we are close the
* limit of bytes we are allowed to copy. */
m = MAX(MIN(COPY_BUFFER_SIZE, max_bytes), m - n);
copied_something = true;
}
if (FLAGS_SET(copy_flags, COPY_VERIFY_LINKED)) {