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Commit 02300bdc9375c868bc31debe6f5d748cd03aa391 introduces a segfault #234

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TomAndrews opened this issue Apr 18, 2018 · 1 comment
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@TomAndrews
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@TomAndrews TomAndrews commented Apr 18, 2018

Description

Commit 02300bd seems to introduce a segfault in my code.

I'm afraid I haven't managed to create a reproducible example, let alone a minimal one, but running various jobs before and after this commit point to it being the culprit.

I think the problem is here:
https://github.com/joshuaulrich/xts/blob/master/src/na.c#L274
If the entire final column is NAs then I think _first will be the length of the whole SEXP [i.e. the last valid index + 1] and so we write off the end of real_result when copying the NAs below.

I will create a PR shortly with what seems to fix my problem.

Thanks for all the good work!

TomAndrews added a commit to TomAndrews/xts that referenced this issue Apr 18, 2018
If a column is entirely NA, then firstNonNACol will return the index
of the first entry of the next column.  In the case where the final
column is entirely NA then this will result in writing off the end of
the allocated result.

Fix by subtracting one from the index in this case.

Hopefully fixes joshuaulrich#234
@joshuaulrich
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@joshuaulrich joshuaulrich commented Apr 18, 2018

Thanks for the report!

Sometimes you can force memory issues like this to surface using gctorture(). Other times you have to use valgrind or a sanitizer. In this case, gctorture() works:

x <- xts::.xts(matrix(c(NA, 1, NA, NA), 2, 2), 1:2)
zoo::na.locf(x)
#                     [,1] [,2]
# 1969-12-31 18:00:01   NA   NA
# 1969-12-31 18:00:02    1   NA
gctorture()
xts:::na.locf.xts(x)

 *** caught segfault ***
address (nil), cause 'unknown'
Segmentation fault (core dumped)

Here's a test that any fix needs to be able to pass:

test.nalocf_last_column_all_NA <- function() {
  nacol <- NCOL(XDAT2)
  for (m in MODES) {
    xdat <- XDAT2
    xdat[,nacol] <- xdat[,nacol] * NA
    storage.mode(xdat) <- m
    zdat <- as.zoo(xdat)

    x <- na.locf(xdat)
    z <- na.locf(zdat)
    checkEquals(x, as.xts(z), check.attributes = TRUE)
  }
}
TomAndrews added a commit to TomAndrews/xts that referenced this issue Apr 18, 2018
If a column is entirely NA, then firstNonNACol will return the index
of the first entry of the next column.  In the case where the final
column is entirely NA then this will result in writing off the end of
the allocated result.

Fix by subtracting one from the index in this case.

Hopefully fixes joshuaulrich#234
@joshuaulrich joshuaulrich added the bug label Apr 18, 2018
TomAndrews added a commit to TomAndrews/xts that referenced this issue Apr 19, 2018
If a column is entirely NA, then firstNonNACol will return the index
of the first entry of the next column.  In the case where the final
column is entirely NA then this will result in writing off the end of
the allocated result.

Fix by subtracting one from the index in this case.

Hopefully fixes joshuaulrich#234
@joshuaulrich joshuaulrich added this to the Release 0.11-0 milestone Jul 30, 2018
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