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render: resize(): don't overflow the number of scrollback lines

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The config system allows setting the scrollback lines to
2**32-1.

However, the total number of grid lines is the scrollback lines plus
the window size, and then rounded *up* to the nearest power of two.

Furthermore, the number of rows is represented with a plain 'int'
throughout the code base.

The largest positive integer that fits in an int is 2**31-1. That
however, is not a power of two.

The largest positive integer, that also is a power of two, that fits
in an int is 2**30, or 1073741824.

Ideally, we'd just cast the line count to a 64-bit integer, and call
__builtin_clzl{,l}() on it, and then take the smallest value of that,
or 2**30. But, for some reason, __builtin_clzl(), and
__builtin_clzll() appears to ignore bits above 32, despite they being
typed to long and long long. Bug?

Instead, ensure we never call __builtin_clz() on anything larger than
2**30.

Closes #1828
This commit is contained in:
Daniel Eklöf 2024-09-09 06:51:10 +02:00
parent 8a4bbbf5cb
commit 1925593a37
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GPG key ID: 5BBD4992C116573F
2 changed files with 34 additions and 4 deletions
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33
render.c
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@ -4369,8 +4369,6 @@ render_resize(struct terminal *term, int width, int height, uint8_t opts)
term->width = width;
term->height = height;
const uint32_t scrollback_lines = term->render.scrollback_lines;
/* Screen rows/cols before resize */
int old_cols = term->cols;
int old_rows = term->rows;
@ -4379,9 +4377,36 @@ render_resize(struct terminal *term, int width, int height, uint8_t opts)
const int new_cols = (term->width - 2 * pad_x) / term->cell_width;
const int new_rows = (term->height - 2 * pad_y) / term->cell_height;
/*
* Requirements for scrollback:
*
* a) total number of rows (visible + scrollback history) must be
* a power of two
* b) must be representable in a plain int (signed)
*
* This means that on a "normal" system, where ints are 32-bit,
* the largest possible scrollback size is 1073741824 (0x40000000,
* 1 << 30).
*
* The largest *signed* int is 2147483647 (0x7fffffff), which is
* *not* a power of two.
*
* Note that these are theoretical limits. Most of the time,
* you'll get a memory allocation failure when trying to allocate
* the grid array.
*/
const unsigned max_scrollback = (INT_MAX >> 1) + 1;
const unsigned scrollback_lines_not_yet_power_of_two =
min((uint64_t)term->render.scrollback_lines + new_rows - 1, max_scrollback);
/* Grid rows/cols after resize */
const int new_normal_grid_rows = 1 << (32 - __builtin_clz(new_rows + scrollback_lines - 1));
const int new_alt_grid_rows = 1 << (32 - __builtin_clz(new_rows));
const int new_normal_grid_rows =
min(1u << (32 - __builtin_clz(scrollback_lines_not_yet_power_of_two)),
max_scrollback);
const int new_alt_grid_rows =
min(1u << (32 - __builtin_clz(new_rows)), max_scrollback);
LOG_DBG("grid rows: %d", new_normal_grid_rows);
xassert(new_cols >= 1);
xassert(new_rows >= 1);